Ah, an old thread I found valuable!
Figured I'd add my experiences here. I recently bought an Aries 1, all original stuff, in great working order. I absolutely loved it - sold my Clearaudio right away.
A friend has an Aries 3, and he loaned me his 300 rpm motor. Darned if it didn't sound better than my 600 rpm. Crazy stuff. I even have an ADS. I felt that the noise floor was lower - maybe the 600 rpm emits electrical noise? And the details were crisper. Maybe because the RF noise was lower, or maybe because the 300 rpm has better speed control, even when handed a clean signal from the ADS.
I know the mantra with VPI is "older is better" but in the case of motors, I think the Aries 3 - era 300 rpm is better than the Aries 1 - era 600 rpm.
Just my $0.02 from a half hour or so of A/B on a couple of tracks I know very well.