Definitely the more support the better; including sports tops at night (you may also find that you need to continue doing this whilst breastfeeding, so as not to leak all over the bed - sorry if TMI!) so you'll get the wear out of them if buying them now.

I also thought I'd mention that if your BBs are growing very fast, consider moisturising heavily (I actually used bio oil on my BBs as well as tummy from early on in pg), as some women get stretchmarks on BBs...I didn't get any despite growing from a C to a G cup! (My sister, who gave me this advice, wasn't so lucky, and has big purple stretchmarks on the top and bottom of both BBs).