Nickel. I'm 60, it's not my only bass, and I play flats. They'll see me out.
I've had stainless fitted on 6 string guitars, and I can really see why that makes sense. Bending strings is a whole new ballgame on stainless, effortlessly smooth.
However, with stainless, the feel does change quite a lot, and it's going to take enough getting used to going from very low frets to new ones, plus I do like the feel of nickel on basses.
Rob