I liked how it was done for 360 tour. There were 3 or 4 tiers. Old timers, so to speak, who were paid members at a certain cut off date. Then it was people who had subbed after the cut off date and before tour announcement, and then noobs who had subscribed after tour announcement. Allowed to spread all fans around a bit better.
Personally, as someone who subscribes every year, tour or no tour, I would love a separate presale for people like me. But as anything, that would piss off a lot of people. No matter what U2 do, they'll piss of lots of people.