My different make bag is a 700, with ratings of Comfort -3C, Limit -10C, Extreme -28C. I'm a bit of a cold sleeper and it's spot on for me in May and September.
Prior to that I had a 650 fill bag with ratings of Comfort +2C, Limt -3C, Extreme -20C, and that was most definitely not warm enough for me.
If you buy the 700 and sometimes find it too warm, being a hot sleeper, you can unzip it at the top and/or bottom, or completely.
If you buy the 500 and don't find it warm enough you can add a sleeping bag liner.