We all saw what happened when Furcal had injury issues last year... A hammy is different from a back, but at the very least, his running ought to be hampered for a little while, and having a hamstring injury can cause extra stress on Furcal's already dangerous back, which could spell further problems.
Furball hasn't had a good year since 2006, so I'd be wary about having him. Aviles has been doing pretty awesomely, and he proved in '08 that he could do it for an entire season.
Aviles it is. Too bad he plays for those lousy Royals.