Interesting. Here's a FIP comp from Fangraphs of those three guys (Sanchez, Jackson and McCarthy), who all happen to be the same age, so their graphs line up nicely. Any guesses which is which?

Now, to be fair, McCarthy has pitched in the best pitcher's park of the bunch the past two years. But even if you add a full half a run to McCarthy's age 27 and 28 FIPs, there's still not a lot to choose between the three of them.
Before I get into why I separate McCarthy from Sanchez and Jackson in this year's free agent crop, I will point out that I'm not terribly jazzed about the idea of signing either as the long term ace of the club. It's just that if the front office is determined to pick up a starting pitcher from free agency this winter, I'd rather it be one of those two. If signing one of Sanchez or Jackson is meant as a stop gap measure until they can either trade for or develop their own ace or is done in conjunction with a belief that Lester will rebound from his poor 2012, then I feel much better about it... assuming they have a good reason for believing Lester will turn it around and aren't just crossing their fingers.
Anyway, let's look beyond FIP and check on their xFIP's.
4.21, 4.60, 5.57, 5.49, 4.76, 3.30, 4.23
4.93, 6.29, 4.39, 4.58, 4.04, 3.25, 3.60
4.20, 4.99, 6.26, 5.24, 4.82, 4.97, 4.32, 3.71, 3.73, 3.79
Line one is McCarthy, two is Sanchez and three is Jackson. One of them does not match up with the other two. McCarthy appears, to me at least, to be a pitcher who has benefited from his home ball park and who cannot stay healthy. I think both Sanchez and Jackson are good enough to be number 2s on a championship caliber team and number 1s on a mediocre or bad team. Either could be the best pitcher on the Red Sox next year, so if the front office is determined to build a competitive team for next season, I'd much rather see them go for one of the latter two than McCarthy.
With Lester, Buchholz, Lackey, Doubront, Morales, DLR, Webster, Barnes, Britton and Workman all on the depth chart to start the year, I just don't see the value of spending money on McCarthy. I can see three guys I'd probably bet on having similar seasons to what he'll do in 2013 right on the major league roster already, and a couple of guys in the minors who have the potential to do the same if given the chance. And that's more of a down view on McCarthy than a super rosy projection for Sox pitchers. I'm just not that high on him.
So I guess what I'm advocating here is not blowing the farm on one pitcher and instead plugging a hole in the rotation with a guy who has a chance to be the best pitcher on the roster next year with eyes toward developing a top of the rotation starter from within.
Whether they get lucky and that ends up being DLR or Barnes continues moving forward or someone younger flies through the ranks like Buttrey or a draft pick from next year or maybe they push hard to sign Otani thinking he's that guy, I believe the best course long term is to hold onto the system's top prospects and build around those that pan out while using money in free agency for guys like Jackson and Sanchez when they're available.
If you can swindle a team like the Mariners for a talent like Hernandez by not giving up any of Bogaerts, Bradley, Barnes or DLR, sure you jump at it. But once you start talking about including those guys, I get a bit apprehensive and I don't see any incentive for Seattle to deal him without getting at least two of those names.