Temp ratings on bags don't mean squat unless independently verified (or at least tested to EN standards). Most cheap bags are not independently rated, can pretty much put whatever number they feel like on there.
Real temp ratings
usually sometimes have a comfort vs. survival rating. The survival rating is just that...you *probably* won't die if it gets down to that temp, but it will be the most miserable night of your life. So plan according to the comfort rating.
Even legitimate ratings are somewhat subjective because everyone sleeps hotter or colder. If you sleep cold then you need to go up a step. E.g. if you're planning on it being in the 20s then get a 0 degree bag.
No bag will live up to its rating if used improperly. Make sure you are cinching it tightly around your face and shoulders, there should be no air gaps. And get into your bag before it gets very cold out. It's way more difficult to get warm once you're cold.
If you run cold and really want to stay warm in cold temps you need a quality down bag. You want something like 800 fill down. Synthetic is heavy relative to the warmth, doesn't compress very well, and in my experience it never quite works as well. But you need to ensure a down bag doesn't get wet, though this shouldn't be a problem since you have a good tent. First order of business when you get to camp should be getting the tent up and the sleeping bags protected inside.
Finally, you don't need to spend $500/bag to get good quality. OutdoorVitals makes quality bags at a reasonable price. Their
0 degree bag ($280) is under 3 lbs and works great for my wife who sleeps very cold. I took the family backpacking this fall in the Northern Rockies, with overnight temps just below freezing and 25 with wind chill, and for the first time ever she was too hot and had to vent the bag.