I had a Champ Teebird, and didnt much care for it. However, after trying a beat DX teebird, I began to see its uses. I picked up an 11x KC Pro now, and I love it. I call it my "birdie disc" because it seems every time I pull it out of the bag, I park it for a deuce.
I've thrown vipers, but not a huge fan of leopards (3 cheetahs in my bag tho), and I'd say the teebird is just easier to throw straight. you can get the viper to hold a straight like, but you really have to crank on it, whereas you can throw the bird nice, easy and clean, with much more accuracy, and without having to fight the disc like you would with the viper, which wants to go left (rhbh)