My league is a standard 12-team redraft league and we are doing 1/2 point ppr this year as well...I'm leaning towards Johnson, but have read cases for both.
We just drew for draft spots today and I have 1st pick overall
Flip a coin lol. Really cant go wrong with either but i would say take Johnson he has less injury history than bell. I have the number 2 pick in my league so my descision is easy.
I'd go with David Johnson. Bell has missed games pretty much every season for one reason or another. Now with him holding out, I could see him coming back a bit out of shape and pulling a hammy.