I think dynamically Rukia would be more fun for Grimmjow in bed. Orihime is like...15? As much as the doujinshi might tell us other wise, I doubt she's that experienced in bed...and would probably just lay there and cry. Which might be fun the first few times, but yeah.
Rukia and Grimmjow have so much in bed potential...She's got decades of experience under her belt...Mmm...
Heh, I do not expect anyone but me to get or like this.