C64DX sounds too closely like C64, people who don't know about the C65 might just think of it as a C64 with some extra features. I think C65 is quite a cool name. Who knows whether they'd have really released it under that name, though. Maybe it would have become the C256 or CX or whatever.
But to most people now, who know about it, it's known as the C65, and I would choose something close to that. And the more I think of it, the better C65+ sounds to my ears (and looks to my eyes).