I would say Bloodstock, The pure size of Wacken makes me tired, Last time there was 2008, the year they sold more tickets then they had space for.
The first night Maiden played, and the stopped letting people into the stage area, because you couldnt move in there. even thou the place is big.
Bloodstock was as someone said more of what Wacken used to be, and by my experience people were nicer overall at Bloodstock. I dont know about the last years but earlier Wacken to me always had the issue of a big part of their staff not knowing any english...
So if I had to pick a festival i would take Bloodstock, they also use to book really good bands. Wacken just book allot of bands. some good some bad and two many playing too many stages at the same time. I prefer Metalcamp in this, they have one big stage and at least 5-6 days of conserts, hopefully 7. But wacken is still a better place if you like strolling around strange camps and meet people from everywhere.
so depending on the approach i dont know anymore.