I voted round. I LOVE a square or emerald cut in colored stones, but with diamond (unless you are buying a colored diamond), you get a lot less sparkle with a square or emerald cut, because there are less facets cut into the stone for those shapes. That's the downside. The upside is that a square or emerald cut stone has to be good quality, because the larger table (top of the stone) will not hide flaws as well.
__________________
"Good taste shouldn't have to cost anything extra." - Mickey Drexler
I like round because I think they sparkle more. I looked at round and square when my mother was buying mine, and we decided that round was "twinklier" - which is my standard with diamonds :) Happy shopping.
I like square. They don't sparkle as much but I think they are so much prettier. I have round ones and while I wear them everyday, I secretly (well, not so secretly, actually) lust after the princess-cut studs my co-worker wears (well, hers are a lot larger than mine too).