Should not make any difference, at least for CC. These days it will work just fine with pretty much any card. That said, of course the details will depend on how complex your projects are and whether you plan to e.g. extensively use hardware-accelerated color corrections and that...
And if I'm running CS6?
[Moving to Premiere Pro Hardware forum.]
Message was edited by: Jim Simon
Look under CS6 supported Nvidia card for GPU.
If you do not have a supported card you need to perform the Hack.
The extra memory comes in handy with certain footage and with After Effects, so to answer the question, the 780M should be a better choice.
I can only find a 3GB version, not a 4GB, and I can't find a price for the 775M, so it is difficult to compare to get the best bang for the buck. If the price difference is not dramatic nd you can go for the better card, I suggest that you do so.