I'd go into warlock first, and then into ninja. One of the things I like at the end is to have everyone able to use swords and good armor. Warlocks can't, no matter how you look at it. That's why I try to have no more than one character in that mode at a time.
Of course, the real answer to your question is to try it both ways and see which one you like. You're the real judge when it comes to your game
