NB v 0-3 mo clothing

Nell • Our niña Thalia Magdalen born August 27, 2015.
FTM mom here; so far I've been buying a little NB size stuff in case my LO is really a little one when she shows up, but how much should I get of NB instead of the 0-3 month size? 
I've been following some guidelines for clothing/layette that I've found online in the hopes of avoiding having too much of any one thing or of amassing piles of stuff I won't need. The sizing strategy is a bit confusing though...