I prefer over the ear to on the ear or IEMs. I wear glasses, and have an unusually large noggin. If I was a hat person I'd know my hat size and tell you.
As for seals: I've read some comments here about frame thickness, and it's a bit more complicated than that. My current glasses have a Nike 4306 frame. The side are thin, but they're also very stiff. At the point where the pads meet them the glasses are about 7mm out from my head without headphones, or 6mm when wearing my DCA Noires. I think flexibility of the glasses impacts seal, too.
Having said all that, I find the DCA Aeon Noires to be the most comfortable headphones I can remember. Is seal better if I take the headphones off? Yes. But it's minor for me, probably owing to the long hair which impacts seal regardless of glasses. I can wear these for hours with no physical discomfort, which is far more import to me than the seal.
I'm currently thinking about giving my HD6xx headphones to somebody for Christmas and replacing them with some model open-back model DCA Aeon. I really like how the HD6xx sound, but they're uncomfortable to wear, especially with glasses.