It depends what you want from the concert. If it's atmosphere then GA, if it's a good view then seats.

I was gutted a few years back at Old Trafford, Manchester as Ticketmaster messed up and said they had accidently sold too many GA tickets so we had to accept seating ones as a compromise. The seats were very close but it was the worst feeling I've had at a Bon Jovi concert as not everyone around you stands up and I just kept looking at the standing thinking gorrrrrd I wish I was down there!!!