They advise not to use blankets on sleepbags as bub cant really kick it off if need iykwim.
I would get a lighter one over a heavier one as you can always throw 2 singlets and a long jumpsuit on under and then get lighter in clothing as it warms up
but if you get a winter winter one you can make it much cooler later on

unless you live somewhere it gets totally freezing.. then you will need 2 summer and winter