I think Stafford is going to win this poll pretty comfortably, but I think it's close between the two, and you could probably get me to vote either way.
Stafford played on worse teams for much of his career, but he also played in a dome and has been playing in Los Angeles for at least half the season the past five years. Big Ben had more talent around him for much of his career, but he also played in rougher weather conditions and had to go up against prime Brady, Manning, Rivers, and a young Mahomes.
The one thing I can definitely say about Big Ben is he would make plays all the time where I thought he was going to take a sack, yet found a way out of it and made a massive play. If there is one edge I'd definitely give to him over Stafford, it's his ability to create and extend plays.