Sure, you can use it as a blanket, therefore you have unlimited venting options. If you get hot, just throw it off or shift its position until you cool off and then pull it over until you get warm again.
The biggest difference will be bulk and weight. According to the EE weight sheet, the 10 degree bag weighs half a pound more than the 40 degree bag. Additionally, the 10 degree will be considerably bulkier than the 40 degreee.
I'd buy both if I were you, but I've got a gear addiction problem so you'd probably be better following someone else's advice.