In all honestly I would take Hosmer as well, but just for argument's sake:
For JUST 2012, there's a decent chance Gio outperforms Hosmer, and he's $1 cheaper. Gio's moving to the National League - facing the pitcher instead of the DH should help his walk rate, which is his biggest problem. He's got an improved team around him, he's just getting into his prime years. He also finished 2011 very, very strong - 1.24 ERA and a sub 1.00 WHIP his last 4 starts. If things break right for him, I could see a low 3's ERA, average WHIP, 200 K's, and mid/high teen wins, making him a very valuable fantasy asset indeed.
Whereas a lot of Hosmer's appeal comes from his long-term value. He came up and never really struggled, which is amazing for his age, but he never really dominated either. It's easy to dream on him being the next .300/30/100/100 1B, but is he going to be that in 2012? Probably not. For just 2012, he'll probably be solid. With 1B, though, the bar is set pretty high.