20mm rails seem normal on that size of machine. Overkill in terms of strength for the Z axis, but when you start looking at the Z design in detail you are probably going to end up with two parallel plates, one fixed and one moving. 15mm rails don't really give you enough room between the plates for a ballnut. Use 20mm and you will need to cut some clearance slots and pockets, but it's a much easier fit. Belt or direct drive won't make any difference to backlash in a ballnut (assuming that you use a decent quality coupling between motor and ballscrew) - it's only the way that the ballscrew is driven and backlash comes down to the quality (=price!) of the components you use which affects the fit between nut and screw.