Who is the best Yankees reliever, Dellin Betances or Aroldis Chapman?

Two of the Yankees' best players are Dellin Betances and Aroldis Chapman. The Yankees have made a habit of employing a pair of dominant relievers at nearly all times the past decade, and this season will be no different. The roster as a whole looks to be middling, but the back of the bullpen appears to be the exception.

Having two such dominant relievers opens the door for plenty of creativity regarding bullpen usage, but in all likelihood, Chapman will be the closer and Betances the primary setup man. It's a conventional formula, and it will probably work just fine. One question that’s hardly been asked, however, is simple: who is better?

By better, I mean the more effective player right now. That means setting aside things like contract terms, which make Betances the more valuable asset, and the fact that only one of the top Yankee relievers has managed to not be accused of domestic violence, which makes Chapman more difficult to root for.

In terms of pure run prevention, the two are remarkably similar. Despite team president Randy Levine's assertion that Betances "doesn't have the stats," his numbers actually match up almost perfectly with the man guaranteed $86 million by Levine and company. Betances may not have saves due to his role, but he does have a 47 ERA- (ERA adjusted for park and league factors) since 2014, which squares almost perfectly with Chapman's figure of 44 over the same span.

How do we choose between these elite relievers who share an equal penchant for suppressing runs? Let’s check the tale of the tape.

The Case for Chapman

The argument that Chapman is the best reliever on the Yankees, and possibly in the entire league, mostly boils down to his ability to do things with a baseball that no one else can. That includes throwing a 105 mph pitch (for a ball!). Chapman might be the best reliever in the game because his arm talent can make him nearly unhittable.

Since 2014, Chapman's FIP- is a minuscule 36, compared to 44 for Betances. He has struck out 44.4% of batters faced over that span, best in the league, while Betances has struck out *just* 40.3%. Chapman has an advantage in terms of simply blowing the ball by batters.

This holds up when looking at the pair's contact rates. Opposing hitters have made contact on just over 60% of their swings against Chapman over the past three years, while they've made contact on about 66% of swings against Betances. Chapman's swinging strike rate has been a startling 19.3%, compared to Betances at 14.5% (all data courtesy of FanGraphs).

On a per-at-bat basis, Chapman looks to hold an edge. Hitters are simply less likely to be able to even get the bat to the ball when they face Chapman.

The Case for Betances

Yet baseball isn't all about per-at-bat efficiency. The scope of one's performance matters. Gary Sanchez was as good as anybody on a per-inning basis last year, but it would obviously be ridiculous to put him above players like Mike Trout, who were just as effective in a broader sample.

Betances might seem to trail Chapman on a micro-level, but zoom out, and it's clear that he has dominated on a larger scale. Betances easily leads major league relievers in innings pitched since 2014 with 247, about 40% more innings pitched than Chapman over that span. In baseball history, there have only been six instances in which a reliever threw at least 80 innings and struck out over 13.5 batters per nine, and two belong to Betances (2014 and 2015, across which he had a 1.45 ERA). That Betances has maintained fielding independent and run prevention figures very nearly as good as Chapman's while carrying such a hefty workload is remarkable.

That volume gives Betances the edge over Chapman in terms of WAR. Betances has totaled 8.5 rWAR since 2014, more than Chapman's 7.1. That should come as no surprise: WAR is a counting metric, and it favors players who play more.

So it would seem that the Betances vs. Chapman debate comes down to a question of volume vs. per-plate-appearance dominance. Who you prefer may simply come down to taste. Do you choose the guy who has possibly the best pure arm in the game’s history, or the guy who provides an unprecedented combination of elite performance and volume?

To me, it seems that if a manager needed to strike out one batter in a sticky situation, Chapman would be his choice, but if he needed two shutout innings of relief, Betances would be the man for the job. The latter job is more strenuous, which is why I lean Betances in this debate, but the Cubs just showed us how much value teams put on the best guy to get one out (they valued him about the same as Gleyber Torres).

There's no easy answer to the question of which kind of player is superior. Chapman and Betances each have cogent arguments to be the best reliever in baseball, and they happen to play on the same team. To really determine the best player, we might just have to see the roles reversed, to see whether Chapman could sustain Betances’ workload, and if Betances could be even more efficient if he was given a smaller burden. Regardless, it’s undeniable the Yankees will again have an incredible late-game combo, even if we might never get the chance to see which member of their elite relief duo is the superior option.