Down bags will loose far less of the rating than synthetic. A synthetic can loose 30 percent in a season. A good quality down bag, you could have for 20 years with little change. I've not really even noticed much difference in rating , as long as they are well made. I have a preference for Western Mountaineering bags, or Feathered Friends, however you do not need to spend that amount. I would look for Goose Down though, and good draft collars, and good zippers which are not always there on less expensive bags ... or I would just buy a Western Mountaineering bag and be done with it.
