I can't speak towards comparing the two amps but I've read a lot from some pretty damned on-top-of-it Stax aficionados and from what I've read, the SRM-323S is considered by many to be one of the best (if not the best, considering the money) Stax amps currently available ... also, one of the most powerful.
You could probably expect a slightly more detailed image with the SRM-323S ... I use one currently (with 404LE's and Koss 950's) and love it. Had a SRM-T1 before and the 323S blows it away (for my preferences, anyway) ...
Thanks for the reply. I'm surprised that the SRM-323S is rated so highly compared to the vacuum tube equipped SRM-T1 and the SRM-006TS costs about £300 more than the SRM-323S. Does the SRM-T1 produce a warmer sound?