The M64 sucks and doesn't even deserve the TNT2 name. It was the "MX" of it's time. In it's day, the only advantage it had over the V2 was 32bit color and the ability to run at a res higher than 1024x768, but that wasn't enough to make up for it's lack of performance. The SLI will walk all over it otherwise. Given a TNT1 or TNT2 M64, I'd take the TNT1 every time.
Anyway, neither is a very viable gaming option these days, but if I had to choose one, I'd guess it'd be the M64 by default since there's still drivers support for it (RIP 3dfx). You can always use it for a work station.