Honestly I'm not a Cardinals fan but I think they have the best shot to win the division.. They have a pitching giant in Wainwright, Carpenter and Garcia. They also have a very good lineup with players like Holliday, Lopez, Rasmus and even Freese when he's healthy for run-support around Albert.
If the Cardinals make a deal to get a top 5 pitcher (not like Lincecum, more like top 5 that are going to be traded), they could easily take the division. Couple that with 4 series' against the Pirates that they'll most likely take every game (would be about 10-12 wins) they have a better chance than the Reds do.
The Reds on the other hand have enormous amounts of pitching. Leake, Wood, Volquez returning soon, Cueto, and even Arroyo to some extent. Votto is a total machine and is likely the MVP at this point. They will get most of their games against the Cubs, Pirates and Astros the rest of the year which are easily winnable. If Stubbs can bring up his OBP to be a half decent lead-off batter that could provide even more R to a team already in the top 5.
The Reds will have to improve on their ERA, ranking 18th in the league and 10th in the NL. Also their 310 walks ranks 22nd in the league and 12th in the NL. They're behind the Pirates sadly.
Obviously the Reds have a chance, but if Scott Rolen starts to cool off they might be in trouble trying to find someone to replace his 57 RBI, with only Stubbs (43) and Bruce (36) behind him. He only has 4 RBI in his last 8 games, a rarity for him and his average has fallen from .312 to .292 since June 12th. The only reason I bring this up is because he's a 1st half hitter (.289 to .277 BA, 708 RBI to 478 RBI) and he might start cooling off.
Votto is definitely in MVP form right now and probably could carry them to the playoffs by himself.