I found the preload on the 12mm axle to be more finicky. I.e., sometimes the hub would feel like it was dragging a bit and other times it would feel like it a bit sloppy. I think the smaller diameter was a part of this - overall the hub felt less stiff, mostly in the sense of just what I'd put under the overarching term "smoothness" and the bearings spin at 25% more, which means a shorter lifespan. Now, my experience with the hub may have been magnified being built into a Zipp 404, which is a stiffer hoop laterally and a more compliant hoop vertically, than a Mavic Open Pro 32.
Basically, when you look at the set-up of the 15mm axle hubs, there were a number of changes made to the preload, axle, and everything else. It's about as significant an overhaul as you could do. I believe the pawl engagement system is the same and the hub shells are the same size, but that's it. When I made the switch, it was very noticeable. The preload is much less finicky, the hub is smoother from a riding perspective (likely due to it being much stiffer and by having less troublesome preload). It just rides better.
There is a cyclingnews article on the new hubs (from last year, before the TdF) that's give all of the metrics. If you really care about me telling you it's exactly 12% stiffer, for example, I'll go and dig up the actual data.
The 15mm axle hub is a good hub. The 12mm axle hub was adequate. I still think the hub could be further improved, but the core electronics are still very expensive, and IMO, people don't appreciate the difference hub quality makes enough. I.e., I don't know that people really want to pay more for a better hub, since it's not something you can see/touch/etc. But it's quite important.
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