I agree with the pavement sentiment. Dancing on concrete for 3 days straight definitely has a toll on your feet. It's not that big of a deal.
What EDC offers over Ultra is the experience. Ultra is just crowds and music. EDC on the other hand is an EDM festival that also has carnival rides. I was able to ride on some of the rides since I always try to get there around 7 when it is pretty empty. Also, dancing from sunset to sunrise is an amazing feeling.