To me, the whole point of MBing is to experiment - it's not the goal, it's the journey.

You have a lot to choose from, I would try them all, and see what you like. Also, why not level them ALL? Each instance is different, in regards to mob, so what works for ramps, may not work for slave pens. Know what I mean?

There's no "best" duo, it's all down to what you like, what you're good at, and how far you're willing to go with them. You admit you like to have lots of alts, so go with that - level all of them, when you hit a wall, swap one out and try another combo. You have choices, which is good - why not set a goal of having one of each class, so you can find out which combo is best, for you?