Bill, i have been told by those that have heard both the II and III that the differences are subtle but significant in an absolute sense. slightly more ease and solidity, slightly deeper and tighter bass on the III. also, a slightly lower noise floor and speed stability with the III. are the differences worth $20k? , $30k?, $40k?
they could be if you can afford it.
as i own a Sirius II i can say it outperforms any other turntable i have heard by a considerable margin. that includes the Basis Debut Vacuum, the Clearaudio Master Reference, and the La Luce Centoventi.....all i considered seriously until i found the Sirius II.
i have not heard the Sirius III. there are 2 main differences between the II and III.
1. direct drive on the III verses belt drive on the II as you have already mentioned.
2. the plinth on the II is Granite, the Plinth on the III is a composite. the advantage of the composite is that it is said to be more neutral and forgiving of some cartridges; the Granite is more "hard" sounding.
there are other less dramatic differences between the II and III in an evolutionary sense but the functioning of both turntables are the same.
i paid less for the Sirius II(used) than i might have paid for 2 of the other(new) turntables i mentioned above.
once you have seen and used a Rockport, all other turntables will seem like crude toys in comparison. i don't say that in any way to criticize or belittle other turntables, many of which sound great. but a Rockport is made to a whole different standard than anything else.