In the Rockies, summer nights will regularly get down close to freezing if not below. Some nights will be 40*F and some will be nearer 20*, and this often depends not just on the weather forecast (not very reliable in mountains) but also on local microclimates and the altitude.
I personally will not go out in either the Rockies or Cascades without a 20*F bag--I get COLD! Actually, the "comfort" (women's) rating for my bag is about 25*F, although I've taken it down to 15*F by wearing all my insulating clothing inside over a vapor barrier suit and having a 75 lb. dog to snuggle up to. A warm sleeping pad helps, too.
If a sleeping bag is not EN13537 rated
, I'd put the temp rating about 10*F higher than that set by the manufacturer's marketing department.
You might want to look at separate sleeping bags that can be zipped together. The budget down bag that is most popular right now (recommended for Boy Scouts on a budget) is the Kelty Cosmic Down 20. I do not know if they have compatible zippers, though--you'd need a right zipper on one bag and a left zipper on the other. You could check that on the Kelty website.
Do get at least your packs from a place that will allow returns--I'd be a bit wary of ebay. As I said, it's like buying shoes sight unseen. Make sure anything you order is returnable (usually you'll have to pay return shipping cost).
Down is much more compressible than synthetics so takes up a lot less pack room. Unlike synthetics, down recovers more quickly and completely from being compressed. Because of this, down bags will outlast synthetics. A good synthetic bag lasts 5-8 years (depending on how much its used) before losing significant loft; a high quality down bag will last a lifetime if properly cared for. Down is, of course, a lot warmer for the weight, which concerns us--as the motto of this site goes, lightweight backpacking is more fun!