I took one look at the RS prices and immediately went for the XL, don't know if yours are similarly priced though
10t sounds a bit small, only 4 engaged, not a lot of leverage. Being an accuracy freak I'd suggest you pre-tension it way beyond the rating for the motor shaft.

This means a lay shaft with bearings either side of the driving pulley. OTOH once you do that you get a second set of pulleys in the equasion so you can beef up the drivers above 10t and get a nice fat shaft through them.