I am looking into the HIWIN Rails - and many people on the forum suggests 20mm because of grease, access to bearings etc. So I take it 20 is good,15mm bad ?

The motors I am looking at can be 3mH if you couple them in series, 12mH in Parallel configuration.

Those drivers are nice, £100 nice - will have to save up for them....

Busy re-designing for the Linear Rails... will post new design within the next week.