Direct drive vs belt vs rim vs idler arm

By the way, Mr. Dover, where did you get an AC motor that doesn't cog? Can you describe it?

The belt creep that Mark Kelly expounds assumes the belt stretches and thins slightly, ergo if the belt doesn't stretch then there is no belt creep.
Japan. Big and round.
There are lots of ways to reducing or eradicating cogging, varying the ratio of slots to poles, angled slots or stators, variable drive applied to the motor windings, running the motor at high speed etc
In the Final Audio it uses an AC synchronous motor with precisely controlled regenerated sine/cosine waves for the motor and variable voltage regulation to optimise the torque applied to the moving high mass/high inertia platter. I can actually dial in cogging via the torque controller if someone likes to listen to that.

Motor speed control circuits have to be designed for the various types of drives. For belt drive tt's the circuits must likely be tuned specifically for the belt compliance/platter mass being used. DD tt's must have altogether different speed control circuits as compared to idler or belt drive. Back to belt drive; I think changing belt types on a tt affects the sound because the spring rate of the belt has changed thereby affecting the dynamic response of the system. The speed control circuits are designed around a specific set of parameters including the belt type. Changing belt types alters the spring rate side of the equation and may make the table sound better or possibly worse. It might be hit or miss. The same goes for adding mass to the platter.
You have to try the Dr. Feickert Analogue iPhone/iPad app. I think it is a killer app. You need a test record with a 3150Hz test tone. You can order one through the maker of this app or in my case, I have a test record with a 3150Hz test tone. This app does three things; it lets you dial in platter speed while the cartridge is tracking in the groove, it analyzes and computes your table's Wow&Flutter (it automatically filters out the record's runout which can accentuate Wow&Flutter) and it charts your platter's speed over time. My particular tt has Wow&Flutter measuring +0.02%/-0.03%. That is within specifications. The speed plot over time shows a small, smooth sinusoidal wave. That might be motor cogging smoothed out by the platter mass, but I think it might be the motor speed controller cycling about the setpoint. I measured the speed of an old Oracle tt and it did a little better than my tt. It had a Wow&Flutter measurement of +0.01%/-0.01%. It also had a smooth, low frequency sine wave for speed vs. time. The Oracle was harder to adjust and dial-in an exact speed of 33 1/3. I would really like to hear from others how their idler drive and direct drive tt's compare with the speed vs time plotted by this app. It might give us some insight into why the different drive systems sound different.

I also meant to mention that by setting platter speed with this iPhone app, it significantly improved the PRAT of my tt. I had been using the little strobe disc for years to set platter speed. The speed was off by a significant amount using the little manufacturer supplied strobe disc.

I respect all of our opinions, and really being happy with one's turntable is all that counts. For me, in practice, there is such a thing as "good enough". The theoretical discussions are a separate bit of play.

Well said, Lewm, and a welcomed reality check in a discussion verging at times toward angel-counting on a pinhead.