I'm sticking with Weeks. Most every young player will experience some kind learning period in the majors and the only way to get better is to play. For now, Yost is letting Weeks make his mistakes and learn from them (hopefully he learns). I think Weeks ceiling is higher than Uggla's and that Uggla will experience some form of the same slump as Weeks is in now.
I know Uggla's had a nice streak recently, but his history screams out mediocre player. We all know Weeks a stud and we know he's locked into the lineup for the rezt of the season. Uggla will be lucky to hold the 2nd spot when Hermida comes back.