I took up the c-system CBA about six months ago. I bought a very cheap and much used two voice 72 bass Hohner with a flat keyboard. The reason that I bought such a low value instrument was that at 70 I was not confident that I would be able to learn to play it.
After a couple of months it was clear that with application and practice I would certainly be able to play well enough to satisfy myself, although the feelings of those within earshot might be different. So I bought another newer Hohner which I hoped would prove lighter and better sounding. This was a Nova with sixty basses. It had a stepped keyboard, which I found to be different, but not sufficient to make much difficulty in adapting between the two.
Having progressed in playing last month I decided to go large, as Macdonald’s has it. I part exchanged both accordions for a 120 bass Paolo Soprani with four voices, innumerable switches, and a casotto. This, apart from its bulk and weight, makes a lovely sound, and has a stepped keyboard. It is a delight to play, although I find that my left shoulder aches after an hour of practice.
I would describe the differences between stepped and flat keyboards as trivial. I also play a number of other instruments, among them diatonic accordions. So very very different my dear! I play both stepped and plain keyboards on the diatonics. If forced to choose I think I’d go for a stepped keyboard, but I think the practical difference is much less of a factor in choosing an instrument than the quality of the sound it makes and the pleasure it gives me in playing it. In any case, the choice of instruments available to buy in the English speaking world is pretty limited. If you find one you like and can afford then go and look at it, have a go at playing it and if it seems to answer your requirements purchase it. You’re a long time dead.
Hope this helps.