Let's look at their 2013 numbers:
Burnett: 191 IP, 3.30 ERA, 9.85 K/9, 3.16 BB/9, 56.5 GB%, 9.1% HR/FB
Arroyo: 202 IP, 3.79 ERA, 5.52 K/9, 1.51 BB/9, 44.4 GB%, 14.0% HR/FB
And their career numbers:
Burnett: 3.99 ERA, 8.34 K/9, 3.65 BB/9, 50.4 GB%, 11.2% HR/FB, 37.5 fWAR
Arroyo: 4.19 ERA, 5.84 K/9, 2.46 BB/9, 40.6 GB%, 10.9% HR/FB, 23.4 fWAR
Burnett and Arroyo are nearly the same age -- Burnett turned 37 four days ago, and Arroyo turns 37 in late February. Burnett, though he used to throw in the mid-90s, still throws between 92-93 mph and is capable of racking up strikeouts. Arroyo, on the other hand, throws around 87 mph and does not record many strikeouts. But he does limit his walks.
Perhaps because of his declining velocity, Arroyo has had to rely more on sinkers and curveballs and less on four-seam fastballs.
|Bronson Arroyo's pitch usage chart|
Burnett's groundball percentages have been over 56% in each of the past two seasons, which would far away be the best in the O's rotation. Last year, the leader in the groundball percentage department for O's starters was Scott Feldman (48.6%), who won't be returning. Chris Tillman, Miguel Gonzalez, Wei-Yin Chen, and Bud Norris are not groundball pitchers, and Kevin Gausman, who may start the season in Norfolk's rotation, probably isn't either. Being a groundball pitcher isn't a requirement to pitch effectively, but it does help, especially in a ballpark like Camden Yards and with Manny Machado and J.J. Hardy on the left side of the infield.
Until the 2011 season, Burnett had primarily been a four-seam fastball/curveball pitcher. But in 2012, he went from throwing his four-seamer 42% of the time to about 22%, and he began throwing his two-seamer/sinker a lot more (from 13% in 2011 to 36%). That trend continued in 2013, and an increase in sinker usage seems to have helped Burnett keep the ball on the ground. Overall, his sinker has been very good, and his curveball has been excellent as well.
|A.J. Burnett's pitch usage chart|
The Orioles should sign Burnett. He strikes more batters out, induces more groundballs, and his home run rate is about the same while allowing about 8% fewer fly balls than Arroyo. Arroyo's advantage comes in the walks department, but that's about it. Arroyo also is seeking a multiyear deal, while 2014 may be the last year Burnett pitches before he retires. He made $16.5 million each from 2009-2013, and it might take something north of $12 or $13 million for him to sign. But for one year, that doesn't seem like such a steep price.
There are cases to be made that the Orioles should be targeting Garza, Jimenez, Santana, or even Masahiro Tanaka instead of older arms like Burnett and Arroyo. (Jon Shepherd wrote about Tanaka back in October, and here are two other calls for the O's to sign Tanaka, which they won't be doing.) I would somewhat agree, especially in Tanaka's case. But the Orioles do not offer long-term deals to pitchers.
Stats and data via FanGraphs and Brooks Baseball.