I also prefer snaps. Zippers are hard for a quick diaper check and I found it a pain to get the second leg (the one without the zipper) in and out. I just didn't snap every button, so it was nbd to snap and unsnap.
In terms of sizing, it depends on your kid. Dd was 7'11 at Birth but grew quickly. She was over 10 pounds by 4 weeks. That plus cding (which added bulk) bumped us out of newborn clothing about three weeks in.