Like I mentioned on your other thread I wouldn't purchase the SD Bag; 3.4lbs is rather heavy by todays standards. I am not sure what your budget is, but Campmor has a sub 2lb +20 degree bag that is highly touted as the best budget bag on the market.

As for a 0 or 20 degree bag-- what kind of camping/backpacking do you do? For three-seasons then a +20 is fine, a zero is overkill and you will overheat in the summer. Secondly if you do winter camping, then opt for the zero, but you will need more than a tarp to protect you during this period of time-- in fact for winter camping your whole set-up would change-- bigger pack, more padded insulation (a single Z-lite wouldn't surfice) and more clothing.