I only know what I've read in all the FAQ's/guides, as well as my own experience with R-A-F (without multiboxing), but I think it depends on how you're leveling. IMO, if you're only doing instances, then the 5 first would probably be faster. If you're doing quests, the 2 would be faster and simpler, allowing you to perhaps do collection quests more easily. From my non-multiboxing RAF experience, the vast majority of the XP is from the quests. At ~40's, myself and my partner are getting Outlands-level XP.