Sell - He stays at least one more season...
I believe he gets his 'guaranteed' 2016 money if he's on the roster at the end of this year... which would finish out his contract (though he's under contract through 2016).
Also, he's only like 2K yards away from the All Time Passing Yards record... nobody active is close to that. He only needs 370 more completions to break the all time pass completions record.
I can't imagine he'd just stop when he's so close.
I think Elway starts look at QB's during this draft and the next though.