Cut quality. If they aren't cut well they won't sparkle. Color and clarity aren't what make a diamond sparkle, cut quality is. Don't skimp on that.
The size difference in mm is going to be negligible between a .5 and .6 when set and on you. You also don't need high clarity or high color(D, E, F or G) for studs. An H, I or J color set would look great as a set of studs, unless you're planning to reset them later as side stones in a ring next to an already-procured higher color stone, in which case, match it within one grade. If they're cut well, they'll be clean, and bright and firey, even on your head.