it depends on the quilt- if it's one you expect to be used ALOT and laundered often i would use a good quality poly- like dream poly, if it's more of a *special* quilt to be shown off more than actually used then a cotton batt would be fine, it also depends on how you want it to look after its laundered,...cotton shrinks more than poly & gives it that 'crinkled look' where as a poly batt will keep it pretty much looking the same after as it does before. I also love wool batts, they wash up well and remain very soft/cuddly/lofty...but they are kind of expensive. also location/climate may play into the equation- if you expect the quilt to be used often & baby lives in a 'chilly' climate- poly is warmer than cotton.