I would say Josh Johnsons, just based on the fact he would be harder to get in the draft than Zimmerman and you have already seen what he is a capable of. Barring injuries Josh Johnson has had awesome stuff, and being the fact that they built a pretty solid team around him he should be able to pick up more wins than Zimmerman as well.
How long do you get to keep the player for though?
Well my keepers are going to be weak at SP. We keep 6 and 1 minor. I was planning on trading either zimmerman or jj plus another hitter to upgrade my starting pitching. Here is what I expect are my keepers: I also still have to decide to keep phillips or arod. Definitely going to keep 2 pitchers (real pitcher heavy league, they score more points). Verlander totaled 996pts compared to ellsbury 686.
Keepers: Teix, Wright, J upton, phillps/arod, zimmerman and JJ
90% of my posts are from complaining about my team
I would rather have Zimmermann but if you are trying to upgrade your SP then you will get a better return on Zimm. Too many people are going to be worried about JJ to deal much for him
"In the beginning the Universe was created. This has made a lot of people very angry and has been widely regarded as a bad move."- Douglas Adams
I would take Johnson. We know how likely he is to break down (VERY), but we also know it won't happen immediately. You could easily keep him for 5 starts, cash in, then leave the headache/risk to another owner.
Josh Johnson for me. He'll most likely throw less innings than Zimmerman, but the quality of those innings make him quite a bit more valuable imo. It's not like Zimmermann is the picture of health either.