You get more exp when not in an honor party when dealing with high level mobs for some reason. Or maybe it's a difference between open world + instance, not sure. I had a friend powerlevel an alt of mine several months ago, and we found grinding in TB that I got less exp in an honor party than out of it, whereas you get more exp if you're honor partying through, say, Kalin's Shrine, than you do in KS without one.
Haven't tried it with the higher zones yet, though, so maybe TB is just weird. It's definitely much more exp than you'll get in one of the 50 instances, though, regardless.