I think AWM2 is the newer, improved, higher quality version. AWM[0] has been around for quite a while so the chips are really inexpensive, therefore they'll be in the budget keyboards. Using AWM2 in the more expensive stuff gives the buying public a tangible justification for the extra outlay required.
I've no idea as to the detailed differences although I would expect things like higher sample rates and/or bit depths, better compression algorithm (if used) and more advanced signal processing.
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John Allcock