My 7-mos-old son likes his blanket, but it is more for cuddling than for warmth. I have discovered him with the blanket over his head, which was scary at first, but he hasn't suffocated yet, and I know he can get it off himself.
Anyway, the sleep sack is obviously better anyway. We have a Grobag, which comes in 3 different weights. The heaviest one they call 2.5, which is "ideal for standard nursery temperatures of 16-20°C (61-69°F)". The sizes also go up to 3-6 years of age, according to the website.
When it is cold where we live, I put my son in a cotton onesie, then a cotton or fleece footed pj, then in the Grobag. I check him and he has never been cold.
Grobag is a British brand, so the weak US dollar makes them a bit pricey, but there is a sale on now, so check them out at http://www.grobag.com/grobags.aspx. They ship to the States. Good luck!