After some more looking.
MS12 preview is better across the board in quality until you get to best then they are the similar. This is also true on 1080p30 type video. Just much less so than hires stills.
MS12 there is little diff from Draft to Good. All are quite good and visually similar. It appears MS12 might be taking advantage of the video scaling hardware that has been in cards since the 90’s.
VP12 seems to do all the work on its own, no hardware offload, and thus has issue with the hires files, and the huge difference in visual quality from preview, to good to best.
The visual quality diffs (prev,good,best) in VP12 are just different algorithms as expected. The hardware scaling is always the same algorithm, typically bilinear.
Still curious, and disappointing, that MS12 has much better playback quality and smoothness than VP12. Might have something to do with the split and multi-cam capabilities in VP12.