My choices have been pretty much determined by the type of fishing I intend to do. I've got an 8' 3 wt. (used with either a 3wt FWF or overlined with RIO's new Nymph line), and an 8'6" TFO 4wt (used with either a 5wt FWF or RIO's Nymph line in a 6wt.)
In the park, if I'm going nymphing, I end up fishing the TFO almost exclusively. I just feel that the St. Croix 3 wt Avid just doesn't have enough backbone to be throwing nymphs around all day... it wears me out and the backcast is less efficient, leading to more bird's nests than I care to Deal with. But it's great for delicate presentation of dries. So I know that if I intend to spend a day throwing nothing but dries, I'll take my little 3wt -- it's just more fun!
Truthfully, though, that TFO has become the workhorse. Probably because I just can't seem to commit to an entire day of either nymphing or dry flies. It may seem silly, but that extra 6" makes a big difference when highsticking.