I am getting one to help in the crib transition. LO is just 2 1/2 months, but almost 16lbs, long for his age and wearing 6 month clothes.... Should I get the small 3-6 months that goes up to 18lbs? Or the large that goes from 18lbs to 21lbs.

ETA: I can't get both and return one since I am getting it used on a BST site.