I have both 6mm and 8mm stainless all thread that I can use for my drive screws.

Is a fine thread (more tpi) not better than a large say 1/2 screw with less threads ?

I would have thought my 6mm 26 odd tpi would give me a lot more acurate cuts or am I looking at this total wrong.

Yes and no. The lower pitch screws do a few things. One is increased force. This at the expense of some speed. Another is increased resolution. Do not confuse this with accuracy. Even with 10 TPI screws and 200 step steppers running at full step you will get .0005" per step. Your machine, even if very tight, will have more slop in it than that, so going to a finer pitch will not improve the accuracy of the machine.

Either screw will work fine. I would use the coarser one, just to get a little more speed.

Eric