I carry both...belt and suspenders. You can only fix one flat with a single tube. I have been in more than one situation where a patch kit saved the day. Last time around it wasn't my bike is was someone else's, and unfortunately the spare tube I had was not the size they needed. But by having a patch kit I was able to help them out.

I always try to keep spares on all my bikes, because a trip to the grocery store may stretch from the 3 mile reality into an all day ride of 15 miles or more if the mood strikes. My wife still hasn't figured out how I can get "lost" between here and the grocery store!

Aaron