Halladay is 33 years old or so, Cain is 7 years younger and career wise their WHIP and ERA are roughly the same. I think it would depend on how the league operates, do you pick all new players every year or do you get a set of keepers from season to season? If you re-draft from scratch every year than I'd probably do this trade as Halladay was awesome in 2010. But, if you carry keepers than I'd much prefer Cain since he is so much younger and you'll be able to have him for years in the future.

Assuming you do have keepers year to year I would almost propose the trade is backwards, personally I'd be pushing the guy giving up Halladay to add in a player, not you adding a player to Cain. Like the other response, I wouldn't even give up Cain straight up for Halladay.