As a user, which tool am I supposed to use.
Whichever you choose. Personally I solely use yum. That eliminates the possibility of the two getting out of sync and there's also only one cache for headers/rpm's. If you run into trouble with the headers and packages getting out of sync, yum has options that erase unused headers, all headers, and/or the cached packages. You can also schedule yum to run automatically each night through system-config-services. You can use the same repos with either so you really only need one. I'm sure others could give their reasons for using up2date, but I think most will agree that sticking to one will cause less headaches.
Wayne Steenburg