I think it's better to have a used quality bag than a cheap Walmart one. So look for one used, and go check it out and if it's clean, then get it. I've done that for a couple of great backpacking bags.

The reason I said to think about how warm you want it is #1 you don't want to be cold ... but #2, you don't want to be sweating too. If you're going to camp in Arches or Moab, for example ... get a fluffy one rated at 30 degrees. If you're going to be camping in the Uintahs ... go for a 15 or 0 degree one, with down that is less fluffy but warmer. Mummy bags are the warmest, but less comfy than square bags.

If you are only going to get one bag, get the warmer one of whatever two options you're considering. Better to not be cold than to be too warm. Also, when they rate a sleeping bag's warmth, it isn't at what temperature you're comfortable, but at which temperature it keeps you alive. So a zero degree bag is really only comfortable down to about 15 or 20 degrees.