Hi Mark,
With regard to the resolution issue :-
I use direct drive on 5mm pitch ballscrews in preference to geared belt drive. Here's why - 'Microstepping'. In my earlier days of X-Y control, microstepping had not even been thought of so steps per mm were at a premium (only for the acceleration / deceleration and mass of the moving parts). Modern electronics means that I get 200 x 8 steps per 5mm or 320 steps/mm which overcomes the acceleration of mass problems and although the microsteps are perhaps not that accurate the zero backlash of the ballscrews more than compensates.
I suggest You go for whichever solution is easiest for you to implement and do not worry too much about steps per mm.
Cheers, Cooperman.