What you going to use it for and where? Are you carrying it or throwing it on the back of the bike, or in a car? How comfortable do you want to be? Are you happy sleeping in all your kit if it's cold and potentially waking up cold in the early hours?
Sleeping bags are a bit of a trade off for price/quality and comfort. Synthetic dries quickly if wet but can be quite bulky. Down is warmer and packs down better but tends to be more expensive.
You can do worse than finding a good outdoor shop and going for a chat if you're unsure. If you've got a good idea then there are bargains to be had sometimes at place like 'Go Outdoors' some of the Vango stuff is quite good and reasonably priced for example.