Bearings are chosen when potential RPM's elevate. Large diameters can over-speed them. Larger bearings take room. Even small bearings can be too big on small shafts. The posts need nicely matched bores and keyways to insure bearings run parallel to workpiece.
Bronze, though they need scrutiny have a bit more than line contact and zero clearance; they will not run with all materials or surface conditions.

All said and done, you are best off equipped with both.