Depends a little on how dirty you want to get your hands (and, I imagine, which is available first?).
CB200 obviously has a little more umph. I personally don't like the tank styling (plenty of others do) but it'd be easy to change out. They're not that rare, so parts are pretty reasonable.
CB125 later models had CDI, so if it's reliable and trustworthy no dealing with points. CB125 used both drum and disk brakes up front depending on the year, but I think both are fine for that displacement and the riding you'll do. It's a thumper, so one less carb to worry about.
I have no idea about production numbers or anything like that, but my impression is there were more CB200s (or there are more CB200s that survived) because I see them for sale much more frequently than CB125s. Happy to be proven wrong there, though.