I have an EMAX and an S900 (which is almost the same as the S950). The EMAX wins hands down, in terms of the grimy sound. The S900 sounds super grimy but the EMAX just takes it to a whole nother level. If you go for the EMAX make sure you get the EMAX I and not the EMAX II, and try to look for a rack version.
The EMAX is also much much easier to use. Its easier and faster to make samples, to edit them, and to map them to the keyboard. And that's very important, the workflow is just way way better.
If you are after the sound quality only, then don't worry about having the memory expansion, just sample stuff into the S950 and then record it into your computer or sample it back to your MPC or your keyboard or whatever you use. If its gonna be your only sampler, then yeah get the S950 because it has the most memory.
I haven't tried the other samplers that formant mentioned, but you also have the Ensoniq Mirage/EPS/ASR-10/ASR-X, the Roland S550/S330/S50, the MPC60 (of course) and the Casio RZ-1. I had an ASR-X that's what I started on, I wouldn't describe it as grimy but it does have a nice sound to it. Sounded more analog than my partner's MPC2K did.