BT/CAT/INT/Whatever in 30 or 40 would be my choice.
R8 is just a glorified collet system.

30 is a nice size for smaller machines, however 40 stuff is more common.
One of my pet hates with my Harrison mill is it's only an INT30, as finding suitable arbors can be a problem, and some of what I've got I had to import from over the pond as no UK supplier could supply them at a reasonable price.