It's a little late since you've already decided (or rather, your sim has), but I'd normally choose by considering how I'd play each sim once they were in the active household. For example, maybe one is Artistic, and I'd be happy to have him paint all day when he's not busy with something else. Or he's Family-Oriented and I'd like the couple to have a bunch of kids, so he'll be in charge of teaching toddler skills and the like. Or he's a Natural Cook and my current sim could burn a salad.
As long as another sim has a good trait or two and no really bad ones, it works for me.
