Well... I sat both motors side by side on the bench today and compared them.
The 1/2 HP
with a pulley runs way smoother than the 3/4 HP (with the bearings replaced)
without a pulley.
Go figure
As for pulley runout.
3/4 HP 0.007 radially, 0.023 axially
1/2 HP 0.003 radially, 0.012 axially
The 3/4 HP which had it's shaft reduced from 5/8" to 1/2" shows no shaft radial runout what so ever, so any pulley runout is in the pulley which I sleeved 5/8" to 1/2". That runout is about the same as what it started out as with a good pulley installed.
Guess it's a no brainer, go with the 1/2 HP motor pulley setup.
Just need to decide if I'd ever use reverse and if so source a switch. What's wired onto the original switch and 3/4 HP motor doesn't appear to be color coded, so I'm a little reluctant to mess with it.
Craig