If you're worried about grainy textures, you want as much detail as possible. 1080P is the "clearest." 1080P breaks your screen down into 1080 lines and the P scans all of them at once across your screen. If you go to 780P, you're only going to have 780 lines on the same size screen. Equaling less detail. Also, don't even bother with I variants of either, because it interlaces your TV, which scans every other line at a time. (think of your shoe laces) Now the contrast ratio of your TV can hugely impact the "crispness" of graphics, but that's a whole other subject
IMO, I have not noticed any grainy textures. I'm running a Toshiba 1080P 120 Hz LCD TV. All though, I've only played MP. So not a lot of time to soak in the graphics. If there is, it's nothing obvious.