what are the advantages/disadvantages of each ball? i am primarily looking for balls that hold their bounce the longest.... it seems these forums everyone likes the Dunlop GP xd balls, but id like to know why. I've always played the standard Penn Extra-Duty (red/black can) tennis balls and never really noticed differences between tennis balls. any detailed comments/ user comparison experiences would be greatly appreciated. Im about to buy a case of balls very soon. Thanks.