Christina:

Since this is a sleeping bag for carrying in a car, go for the colder weather capability unless the space required for a warmer bag is an issue. You can always open the bag if it is too warm, but you can do nothing if the weather is colder than expected and you have a thinly insulated sleeping bag.

My personal preference is synthetics because they require less care. If you buy from someone like LL Bean, they give you a lifetime warranty for most of their products so that if the bag loses loft from storage, they will replace it free.

Good luck!

Bountyhunter