Weight - there is a 25 pound difference between the T5 and Tremec 3550/TKO - about the same weight as 4 gallons of gasoline. The 25 pounds isn't going to make a measureable difference in a 5 speed car.
My thoughts so far have me leaning towards a GFT5. My car is primarily street and though I believe that the TKO with it's stronger case is going to be more durable, I don't see my ever having enough traction to actually break the T5 case with street tires.