Ok, if I were to be completely and brutally honest - I don't like that grey one at all. I don't know what it is, but I don't like it one bit. :-| Sorry.
The khaki one is definately more beachy, it's true... and thats probably what Aaron will end up wearing at our wedding, but I agree with Sabina... on a beach, the grey just kind of pops - in a good way! If it were a different grey suit, maybe I'd vote for it then.
My vote for now, is for khaki! (sorry if that was too harsh!!)