We know about the SB6183 not supporting IPv6 at this point. Which modems support both IPv6 and 300/20 service?
All the MAXX modems can support IPv6.
As noted in the Approved Modem List thread you can refer to the lease or buy page until the list is updated.
I've used the 5370, CM600, 6183 and 6190. Of those the 5370 is the cheapest. The CM500 is about the same price and although the old list only had the CM400 I would expect the CM500 to be on the new list.
Reasons to pick one over the would be status reports (most don't report total packets), price or the spectrum analyzer in the 6183.
Don't spend any money on modems, leases are going away,
The exact nature and timing of post purchase changes is unknown it could be months or years. You'd have to divide the purchase price by the monthly fee and make a guess about time to Charter-fication.
Pesonally I hope the push to reduce unecessary involuntary fees (visible in the case of a TWC "lease" or invisible in the case of Charter providing a modem for "free") will broaden to include modems.