I would be weary of only having pockets to be honist...i only bought BG's with my daughter and they lasted us untill she was 18 months but then she outgrew them...
The only thing with pockets/one size nappies is that they may be way to big on a newborn depending on their weight/shape etc when born...and also depending on when your planning to start using cloth.
If i was going to buy one one type of nappy for a newborn then i would fo with a 2 parter such as little lambs and team them with motherease or thirsties wraps as they offer so much more protection, are much more leak proof etc than pockets in my opinion
Good luck and congratulations!!