From what i've heard, the Blu Thunder is the best of both worlds. It basically a Pro 5.0 and Steeda Tri-Ax put together. After all it was designed by the same guy. It's also about $40-$50 cheaper.
i gotta disagree with the crowd on this one...i have driven many stangs two or three with the triax...it is a vary nice feeling shifter, but my old 5.0 had a pro 5.0 on it and it is the best shifter i have ever driven with/felt, it completely changed the feel of the car, short throw, but not to short, and when you quick shift it just went in so smothly, i never missed a shift with it (cept for once when i backed the adjuster out too much, that was a bitch). i still have the pro 5.0 and it is deffinately going on my next stang. i cannot stress how much i love this shifter...i guess you will have to go out and drive one to see, but then again it is all in the preferance of the user.
IMO they're all pretty much the same. Buy whatever you can find the best deal on. Some peopl will say that the Pro 5.0 is better because it has a steel handle and stop ring, but unless you're beating the crap out of your shifter (which isn't the best idea anyway) you don't need that.