The general consensus is that goose down is ideal for colder temperatures, as it provides the most insulation. The problem with down comes when the sleeping bag gets wet. Essentially, all insulating properties are rendered useless when it gets wet, which is why synthetic sleeping bags are better in that case.
There are also ways you can increase your sleeping bag's temperature rating - with an overbag (basically a bigger sleeping bag that fits overtop), or a sleeping bag liner. Just remember that temperature ratings are given assuming the use of a ground pad, so whether or not you use one should also affect your decision.