I agree with the full-sized spare, especially if you do a lot of traveling. With the donut, if time came to use it, you don't have the same safe handling that you get with a full-sized tire. If the front goes flat, and you still have a long ways to go, you'll have to do two tire changes on the road to put the donut on the rear.
It's happened to me. I once got a front flat with four hours left in an interstate trip. This was on a late Sunday afternoon. I certainly didn't want the donut on the front of my FWD vehicle.
Unfortunately, the trunk wan't designed for a full-sized spare to fit in the same spot as the donut. If your car is designed for a one-for-one substitution donut-for-full-sized-spare, then I recommend going with a full-sized spare.
When time comes for four new tires on the first tire change, I keep the best used tire, buy a full-sized rim from the salvage yard, and use that combination to replace the donut.