I mostly use size 2. I have a few size 1 & 3's that I've acquired from who knows where, but the 2's seem to be the best -- 1's are so small it takes many more of them, 3's are so big it can make it more difficult to move around them as you quilt so you end up with wide open spaces as you prepare to quilt an area.
The Brass ones are like 2-3x the price of the regular silver colored ones. I'm sure they're probably worth it in the long run, but I usually go from basted to quilted in a few weeks & I've yet to have issues in my climate with rusting. If you can spring for the Brass pins, go for it ... otherwise just wait to baste until you're ready to start quilting & you should be fine. And yes, you need WAY more than you would imagine. I think I went through over 200 just on a baby quilt, basted a fist's width apart.