I have done little bike camping, but have backpacked a lot. This is not a difficult choice. I would go with a 20 degree F bag, which is very common and therefore there are a lot of choices. This is also what is recommended... buy a bag that is 10 degrees F colder that the temps that you expect to encounter. Also keep in mind that the temp ratings are based on having a good quality ground pad for insulation (and comfort).
The main decision is down or synthetic. down is lighter and more compressible, but you can't get it wet. And down is much more expensive. Synthetic is cheaper and bulkier/heavier, but keeps you warm when wet.