Thunder is decent, but if you had to choose between Thunder and Nitro, pick the latter.
Bad: Weak gameplay; Sparse commentary; Too fast frame-rate in single matches; Too slow frame-rate when you add wrestlers to matches
I'll admit it. I like Thunder. I think Thunder is a decent game. But for some reason, and I can't put my finger on it, I prefer Nitro. The gameplay is pretty much the only real thing to blame. It's basically the same gameplay as Nitro, but with the faster pace, and with the totally unfair Test of Strength move, Thunder, I believe, pales in comparison to Nitro.
The fast frame rate is one of the problems. I also prefer the PS1 version of Nitro over the N64 version. Why? The pace. It looks super-unrealistic when you perform a piledriver in two seconds. While I like that Thunder isn't as slow as Nitro when you have a tag match or something, it's still slow. It's weird, I complain about the ultra fast pace, then the really slow pace.
Still, aside from the atrocious gameplay, I still like playing it. Thunder is chock full of arenas, characters, and modes. You can compete for titles, and you can even change the character's alliegance (with the exception of the THQ, Inland, and Wacky characters). You can pick WCW, nWo Hollywood, nWo Wolfpac, The Horsemen, and Raven's Flock.
In the end, Thunder's is a decent game. It's full of content, but it doesn't have the solid gameplay to make a great game.