if you are cold sleeping on a thermarest smac, then the big agnes or the exped may not be for you. a rubber casing full of air does not conserve warmth - far from it. you are better off sleeping on a thermarest with a blue foamy underneath (or two). then, you will definitely be insulated from the winter cold
I have the exped7 and I've slept on it with a thermarest underneath, on a canoe trip; and with just the exped. the solo solution is much cooler at night - and I have not tried it yet in the winter.
so it all depends on how much you want to carry. Frankly, the exped weighs *nothing*, so why not shoulder your old trusty thermarest, drop a tiny Exped into your pack (they pack so small!) and try a hybrid approach.
Buy your exped from a retailer that accepts returns (REI, MEC, there may be others) and see how you feel in September or October. If it's cold then, then it *will* be colder when the snows come. If you are not happy - return the item. Keep it in good condition just in case.