Theoretically, a smaller detachment would be easier to get noticed and have more leadership opportunities but they are also limited because of their size. Since you are prior-enlisted, I'd say pick a college that you think you'll have a great time at! This is your opportunity to have a college experience. If you could be happy with your college experience, you'll be happy in your ROTC detachment. Plus, since you're prior enlisted, I'm sure you'll do fine getting noticed, but just try to take on leadership positions.