Even if you use 1-1 belt drive, it can let you tuck the motors almost entirely within the machine envelope - reduce space and remove vulnerable sticky-out bits. There is also a feeling that it can reduce resonance effects although I have no experience of this myself. I went belt drive for space reduction, plus thought that it would allow me to change stepper-ballscrew ratio if needed. I found that this was one design decision I did get right, though, and I have stayed with 1-1. Works well for me (similar size machine).