Believe it or not, I voted Russell Wilson.
Fields is a reclamation project. A rehabilitative prospect that needs retooling. Russell is a classic stopgap that, if schemed for properly, can have limited success. Likely no postseason ceiling to speak of, but you don't lose anything by throwing him in.
Let Fields practice for a year with their coaching staff and sit on gameday as the backup. Encourage him to embrace the role of being a student of the game. Try to shift his perspective on the sport as a signal caller while working on his fundamentals over the year. Russell has been around the block and can handle being the starter. He's reached his ceiling. To throw Fields in now is to lose an opportunity to fix some of his more glaring inadequacies.
Football in the modern day is all about the quarterback. Your success hinges on making your quarterbacks job easier. You want to build him a nest in terms of a supporting cast and build up his momentum and confidence by ensuring you're not starting him off with more than he can handle. Development is progressive and quarterbacks require nurturing.
Hopefully for Pittsburgh, in a year they'll have a rehabilitated Justin Fields with improved fundamental mechanics, restored confidence, and enough support to guardrail him as he tries to rebuild his career. They'll be able to craft a great rushing attack using his athleticism and scheme up the passing game to highlight his strengths. Until then, let Russ have his go and see what he can do.
I understand the thought here, but I simply don't think Fields is a reclamation project at this point.
I think Fields is who he is. You either feel like you can place enough guard rails around the guy (both in scheme and personnel) to take advantage of what he does well, or you don't. I'm not sure a year of being a backup is enough to fix that.
The issue here is: Fields is like washed Russell Wilson in a young Wilson's body only he's more willing to throw INTs than current Russ.
I don't think either of these guys is the starter in Pittsburgh (or anywhere) next year, so the issue is 'win now' and I think that's a legitimate coin flip: Do you go with the slower, older checkdown machine who's safer with the ball? Or the younger, faster checkdown machine who might be able to break a couple of crazy scrambles?
And that's got to be based on what the makeup of the rest of your team looks like, I think, and I don't know enough about Pittsburgh's OL and offensive skill players to make that call--nor do I know what the locker room chemistry is between each QB and the rest of the team.