OK, Iíll put in my 2 cents worth.
We are talking a hobby machine here right?
As you noted 2 Start screws will give you twice the speed. I believe that accuracy from the screws will not be a factor here; either type will give you very good accuracy at this level. Not having seen your design, but Iíll guess that other things will cause your inaccuracies (I could be wrong).
Speed is also probably not a factor with this small of a machine. With this size motor, letís assume a speed of 60ipm for 1 start and 120ipm for 2 start. 60ipm is 1 inch per second. With a table of 12 inches, you get 12 seconds (vs 6 seconds) of rapid speed to transverse the table. This is maybe not a problem here, but in a larger router, speed helps.
A 2 start screw can spin at half the speed of a 1 start screw and get the same speed. This is important when trying to reduce whip, but again, with a small machine, whip should not be a problem.
So with those problems addressed, you come down to lead nuts and mounting. 1 start screws are easier in that you can make your own lead nut with a piece of the lead screw made into a tap, and then just use Delrin or a chunk of plastic cutting board. You can also buy nuts to use to mount the bearings.
2 start screws will probably require you to purchase a lead nut, and use collars or turn down the screw on a lathe for mounting.
SoÖ 1 start are easier and slower, 2 start are a bit more complicated, are really cool, and faster. (And donít underestimate faster).