Defense, get at least 60. Since summoning, at 99, would only level you 12 lvls, from what it appears, and since defense also levels you very slowly, it would be a nice combination. That's what i would think, you just stand there tanking their hits, while your 'summon' kills them.
OMG! I agreed 100%! Every update, this game gets more, and more like another game, WOW, Diablo 2, Guild Wars, I'm tempted to go back to diablo 2. It's so far, one of the most expensive skills to level. I think it's like runecrafting, it's fairly slow, and is rather expensive.