I prefer the styling of the new version, though I did run into a bug when attempting to log in that require me to login on the old version before switching over. All well.
Personally I like the classic version on devices other than my phone (which I never even use Cplusplus.com on because who the heck does C++ programming on a phone?)
The problem with [small], [sub] and [sup] seems to be fixed now. That's great!
Unfortunately there is still some problems with the "preview" with things like [code] and [quote] not being displayed the same way as they will be in the forums. To make it worse they have even started to use the same (or at least very similar) faulty preview for the old version.
Another question, how do you create a new topic? In the old version there is a button that says "new topic" at the top but there doesn't seem to be one for the new "mobile" version. I still prefer the old version, the only reason I noticed was because I was going to check if the long standing bug of not being able to use the formatting buttons when creating a new topic had been fixed. If it hasn't then now perhaps is a good time to do so? (for both versions, assuming they plan to keep the old version)