Yes, Hoffman matched Rivera's ability to convert saves (they both did so at an 89 percent rate over the length of their careers, a better rate than any other closer with 300 or more career saves) and Hoffman had more 20-save seasons than any other pitcher with 15 to Rivera's 14, but the statistical similarities all but end right there.
Rivera has converted more than twice as many saves of more than three outs (115 to Hoffman's 55), and crushes Hoffman in park-adjusted career ERA+ 204 to 141 (with 100 being league average). Having just signed a new two-year contract, Rivera should match Hoffman with a 15th 20-save campaign this season and seems likely to pick up the 43 saves he needs to break Hoffman's career saves record no later than 2012.
Rivera also has a big lead among relievers in most big-picture advanced stats including both versions of Wins Above Replacement (WAR, hereafter referred to as bWAR for Baseball-Reference's version and fWAR for FanGraph's), Baseball Prospectus's Wins Above Replacement Player (WARP), and BP's WXRL (roughly Wins eXpected above Replacement, adjusted for Leverage, a cumulative win-expectancy-based statistic for relief pitchers). Given all of that, the more pertinent question is whether or not Hoffman was the second-best relief pitcher in baseball history, and if not, where does he rank on that list?
Scanning those advanced stats as a group, five names continue to pop up towards the top of those lists behind Rivera: Hall of Famers Hoyt Wilhelm and Goose Gossage, Lee Smith, the man whose career saves record Hoffman broke in 2006, and Hoffman and Billy Wagner, both of whom have just retired this offseason. Noteworthy absences from that quintet are three of the five relief pitchers already in the Hall of Fame, Dennis Eckersley, Bruce Sutter and Rollie Fingers. Eckersley quite simply didn't spend enough of his career as a premium reliever to merit mention. Though he was dominant for five seasons (1988-1992), he posted a 4.05 ERA over the next five before fading away. He ranks 20th all-time in WXRL, and his bWAR for the relief portion of his career ranks down around 45th among relief pitchers, tied with the likes of Gene Garber and Greg Minton. Sutter similarly had just seven seasons with more than 20 saves and an ERA below 4.00. He ranks 14th in WXRL and ninth in bWAR. Fingers saved 341 games, which stood as the career saves record for all but one year between Wilhelm and Smith, and still holds the record for saves of more than three outs with 201 (Sutter ranks third in that category), but he ranks sixth on the WXRL list, just barely ahead of Wilhelm, and fails to crack the top five in any of the wins above replacement stats.
Getting back to our Big Six of Rivera, Wilhelm, Gossage, Smith, Hoffman, and Wagner, we've already established Rivera as the best reliever in baseball history. As for the rest, we'll start with how Hoffman stacks up against Wagner, his other contemporary. Though Wagner has a decided advantage in ERA+ and was one of the great strikeout pitchers in the game's history -- his career rate of 11.9 strikeouts per nine inning ranks behind only Brad Lidge's 12.0 among pitchers with 500 innings pitched (Hoffman's 9.36 K/9 ranks 28th) -- Hoffman holds the edge in save percentage (89 to 86) and saves of more than three outs (55 to 36). He also ranks ahead of Wagner in three of those four advanced stats, trailing him only in fWAR, which produces the leader board least like the other three because it's limited to the period since 1980. It's safe to say, then, that Hoffman was the second-best relief pitcher of his era, behind Rivera but ahead of Wagner. Hoffman also holds a clear edge over Smith, as well, with Smith's only meaningful advantage coming in saves of more than three outs, which, again, had more to do with when he pitched than how he pitched.
Comparing Hoffman to Gossage and Wilhelm, however, is more difficult because of the manner in which each of those pitchers was used. During his nine-year peak, Gossage averaged 93 innings per season. Take out 1979, which he missed most of because of a thumb injury, and the strike-shortened 1981 and his average jumps up to 104 innings in his seven full seasons during that stretch. Gossage also threw 141 2/3 innings of pure relief in 1975, his first full season as a bullpen stopper. Wilhelm, meanwhile, pitched most of his career before the saves statistic was invented in 1969 and surpassed 100 innings of relief work eight times in his first 14 seasons. By comparison, Hoffman averaged less than 63 innings pitched in his 15 20-save seasons.
Hoffman's comparatively light workloads play a large part in his his less-than-convincing showings in the wins above replacement stats, all of which are cumulative, and none of which rank him higher than fourth among relief pitchers. Hoffman, who never made a major league start, threw just 1,089 1/3 innings in his career, 30 percent fewer than the amount of relief innings thrown by Gossage, and just 58 percent as many relief innings as Wilhelm. However, WXRL, which is also cumulative but is weighted by the importance of the game situation, does rank Hoffman second (albeit distantly) to Rivera but ahead of the third-place Gossage while bumping Wilhelm all the way down to seventh. Similarly, Hoffman's poor showing on the list of career saves of more than three outs (he's 72nd) has more to do with the usage patterns for closers during his career than anything else. In the 18 seasons since his rookie year of 1993, Hoffman ranks second to only Rivera in saves of more than three outs.
As a result, where you rank Hoffman relative to Wilhelm and Gossage really depends on whether you prefer a lock-down, three-out closer, or an old-school multi-inning fireman with somewhat less consistent results. It's hard to ignore the fact that Wilhelm threw nearly twice as many innings of relief while also posting a superior ERA+ (Hoffman ranks 12th in ERA+ among relievers with 500 or more innings pitched, below current Reds closer Francisco Cordero and former Blue Jays and Angels set-up man Mark Eichhorn) and converting far more than twice as many saves of more than three outs. Gossage ranks second all-time in the latter category and is second to Rivera in bWAR among relievers if you eliminate the seasons Wilhelm and Gossage spent as starters (1959 for Wilhelm, 1976 for Gossage). Wilhelm, meanwhile, leads in WARP among relievers. Hoffman leads the trio in saves, save percentage, fWAR, WXRL, K/9, K/BB, and WHIP, but, again, with just 58 percent as many relief innings pitched as Wilhelm and 30 percent fewer than Gossage.
Each of the three had a dominant run of nine consecutive seasons. If we narrow our focus to those seasons alone, we get the following:
That peak performance gives Gossage a clear edge, and Wilhelm's combined advantage in innings and adjusted ERA is just too much to look past despite Hoffman's higher-leverage work. As a result, we get a list of the six best relievers in baseball history that looks like this:
1. Mariano Rivera
2. Goose Gossage
3. Hoyt Wilhelm
4. Trevor Hoffman
5. Billy Wagner
6. Lee Smith
Hoffman's position on that list, as well as his soon-to-be second place standing on the all-time saves list, should hold for years to come. It seems we have finally reached the point at which a closers' ability to accumulate saves has maxed out. Yes, Francisco Rodriguez just set the single-season mark in 2008 with 62, but Bobby Thigpen's record of 57 had stood since 1990, and other than Rodriguez, who has 268 career saves, there is no active closer under the age of 30 with more than 200 saves to his name, which is still only a third of the way to Hoffman's record total. More significantly, the consistency and longevity Hoffman displayed across his 18-year career (except for 2003, which was lost to a pair of shoulder surgeries) has not become any more common among relief aces. The Hall of Fame doesn't need to include closers the way it needs to included both catchers and second basemen, but if it does include closers, it needs to have Trevor Hoffman.