In short: Moderate. I’ll be brief here because I’ve got meetings all afternoon.
Per the New York Post, the Mets have interest in Ross and view him as a potential everyday player, but see Hairston as more of a platoon outfielder.
Over the course of their careers, Ross has only marginally outhit Hairston against right-handers: Ross has a .727 OPS in the split to Hairston’s .704. But it’s probably telling that while the two have almost exactly the same number of appearances against left-handed pitchers, Ross has nearly twice as many as Hairston against righties. So it could be that Hairston’s numbers against righties are a bit inflated by a lack of exposure to the toughest right-handers.
Other than that, they’re pretty similar players. Ross is seven months younger than Hairston. They’re both adequate defenders in the outfield corners who can fill in at center in a pinch. They both hit for power, but Ross gets on base a bit more. And, again, Ross has had slightly more success against and a lot more exposure to right-handers.
Of course, signing either would give the Mets only one capable right-handed hitting outfielder at the Major League level. It’s unclear how much they’ll cost and how much the Mets have to spend, but bringing in both to join Kirk Nieuwenhuis, Mike Baxter and Lucas Duda could allow them to cobble together a decent outfield by carefully managing platoons and defensive alignments. It might get a little hairy on defense sometimes, but you start Duda when the ground-ball pitchers are on the mound and so on.
The end. Now, the meetings.