TrainTrac, new Busch is nice--but it, like most new parks, pales in comparison to either Wrigley or Fenway. Not even the most zealous Cardinals fan would try to claim otherwise. (And for the record, Busch has only been home to one world championship team--Fenway's had six.)
Frank--The two are very similar, but I give the edge to Wrigley. Fenway has far more obstructed-view seats, and the vomitoriums (yes, that's what they're called, in all stadiums, look it up) are almost dangerously narrow. But they're both absolutely incredible parks.
See my page of reviews from when I saw a game in every MLB stadium during the summer of 2002. As you'll see, I ranked Wrigley third and Fenway fourth, beat out only by the stadiums in San Francisco and Pittsburgh. (And yes, TrainTrac, I have been to the new Busch since then. Superb park--but not on a par with those four.)