Great point. F0r 2012 You can't take Castro, certainly not over Alexei.
Mr. Bill James says that Castro is going to be better offensively than Alexei this season, by a moderately wide .22 margin in wOBA.
Fielding-wise, yes, Alexei is better. But Castro certainly has an argument, especially if you care about Average, OBP, etc. He is simply superior offensively, sabermetrically and otherwise.
Bill James projects a .795 OPS for Castro, and a .727 for Alexei Ramirez.
Ramirez has Castro beat on slugging and defense though.
But let's not forget that Castro did improve his defense after the all-star break and went a couple months without making an error. He was called up to be a defensive improvement over Theriot, but ended up being a bigger impact offensively.
That said, if you need a 1-year SS, you
probably take Alexei. But if you are looking for a longer-term option, it is Castro by a pretty wide margin. In fact, the only way you take Alexei is if you completely ignore salary, contract details, age, future production, etc.