Some rambling thoughts...
As the mass of a platter rises, inevitably so will the likelihood of bearing noise. They have to go hand in hand. Magnetic levitation may help if it can be implemented without lumps & bumps in the field although I'd still be happier to keep a slight mechanical ground path in the bearing.
The only source
of constant speed in a turntable is the motor. "Removing the motor from the equation" by any form of decoupling diminishes the chance that the record can be rotated at a constant speed past the stylus. Whether this is achieved by mounting the motor in a compliant (at audio frequencies) material or by decoupling thesuspended mass with springs and a compliant belt the effect is the same. Yes there may be improvedsignal/noise but somehow a believable sense of progression can belost.
Using a high mass platter (particularly with an external flywheel) and a thread or tape drive gives the record's movement past the stylus some chance of following the motor. However, a high mass deck will usually be solid andits performance will be affected by what supports it, as the stylus not just reading the groove but the structure supporting it too.
A motor is still a transducer. It will try to follow any hash or noise in the power supply that feeds it not just the dc or ac supply it gets. It always amuses me to read the NAS website about the benefits of heavy platters smoothing out speed fluctuations and then alongside that a recommendation for the 'wave mechanic' power supply.Despite the great mass, the platter is still sensitive to fluctuations in the motor's pull!
For anyone interested here's an example of perhaps the most extreme high mass TT I've read about http://www.aca.gr/paper37.htm
Of course speed constancy isn't everything. CD hasperfect pitch stability but might be said to be flawed elsewhere. It does help, but with good engineering comparable levels of wow & flutter can be achieved by either method. The Continuum turntable is belt driven and yet produces (apparently) some of the most constant speedmeasurements yet recorded.
My preference for DD turntables derives more from the elegance of the solution (if done well) and the possibility of keeping the thing to a manageable size. One of the best turntables I've ever heard was the belt driven, high mass, Morsiani which has a similarly powerful & calm (no sense of a record playing) presentation that the better DD's have. Doubtless the big Micro's also had this quality.
Having owned a couple of idler driven Garrards in the past (and having heard several others) I'm not convinced they, however well sorted, quite reach the same heights although it would be interesting to hear a modern take on the approach with new materials and better motor technology/control applied. They certainly can be made to sound more expressive than the more common Vilchur based suspended subchassis designs that have been so prevalent during the last 30 years.