Their numbers are fairly even since Cespedes came into the league in 2012, but Davis has shown he can be the better offensive player of the two. I don't think Davis is worth $70 million more than Cespedes, but I'd take Davis over Cespedes if the contracts were equal.
While Davis has had a couple of better years than Cespedes, he has also shown tremendous variance between 2012 and 2015. Furthermore, Chris Davis has several red flags that look poorly when predicting future performance.
Normally, there is very little correlation between strikeout rate/swinging strike rate and hitting production. However, when you examine the tail of the distribution (i.e. hitters that strikeout more than 30 percent of the time and/or swing and miss at more than 15 percent of pitches), you see a disturbing trend; hitting production falls off a cliff in the early thirties. There aren't a lot of hitters in this category (i.e. excluding pitchers), so in a large sample size, the lack of correlation between hitting production and strikeout rate wouldn't be affected. The best comparisons are probably Russell Branyan, Josh Hamilton, and Ryan Howard. They are all power hitters who became ineffective by 31 or 32.
Chris Davis has an advantage in that he hasn't had major injury problems, yet; nevertheless, Cespedes may be the safer bet for performing better as a hitter over the next 5 years.
EDIT: For the record, I think this may become the worst deal made in 2015, and the deal might hamstring the Orioles resurgence.
