For starters, DON'T expect anything like Google Mars. The reason - any spacecraft in orbit around Mars is severely limitedin the bandwidth that it can transmit to Earth, and so it would be literally decades in orbit before full planetary coverage was achieved.

I would expect MRO high-res pictures to look about "twice" as good as MOC images - or at least to have what looks like about twice the on-ground resolution. They'll be better, but Mars-shatteringly so. Another step forward, but unlikely to be "dramatic", imho.

As for a comparison between Mars Express and MRO, that's going to be a more subjective thing. It depends if you like large acreage (MEx) or good detail (MRO), and the simulated 3D views from MEx will not be available, so far as I know, from MRO.