If the realm is solid coded and there are no major bugs in quest chains, a x3 exp rate will make you enjoy far more content of the game without leveling being as tiresome as it was on bc retail. About the x5 exp rate, I'm not sure if people agree with my opinion, but in BC, Blizzard offered us the most beautiful, enjoyable, well-designed map to play, Outland, one which you can't fully enjoy it with a x5 exp rate, I mean, you will exceed the zone level, you can play the content. But you won't have the thrust to do so, and even if you do, you will find it easy and not as entertaining as it could be, due to the lack of people playing that content.
Regards.