Not exactly a fair question, since the only way to have played either one is to have attended one of the events where the demos were available and wait through the long-a$$ lines to get your five minutes. I was at Blizzcon '07, and I decided I had better things to do than spend the whole day in line...
That said, I'm looking forward to Diablo 3 much more than Starcraft 2. I've never been a fan of real-time strategy games, and SC never really clicked with me. Diablo 2, on the other hand, is one of my favorite games of all time. I still dig out the old discs at least once a year. I'm really looking forward to seeing the new story...