Stayed at both. I would highly recommend the Grand Floridian. We stayed there this past August and the entire family absolutely loved everything about this resort. It kind of surprised me because we always thought it was "out of our league" as we are laid back, shorts and tee shirt kind of family. Well, we fit right in! The lobby is GEORGOUS! You can literally sit in the lobby and people watch for hours. We stayed in Big Pine Key which was located in between the two wonderful pools. We especially enjoyed the themed pool. There is nothing better than swimming and watching monorails wisk by!
We stayed at the Contemporary tower (MK side) and we will never stay there again. We were located on the loading dock side and we would hear trucks backing up (beep, beep, beep) throughout the night. We were not able to get a good night's sleep. I think if you stayed closer to the MK side you would be OK though.
Go with the Grand - you won't regret it!