Well, when you want to look at "most improved" you first have to decide if a team improved itself better in the long run or the short run. And since the OP is talking about the end of next season, I assume he means the short run. Long run factors more than just the Draft and FA. it also includes keeping/ re-signing/ extending your good players, continuity, a good system, player development, etc. If you don't have that stuff, then you don't even have a long term plan and nothing will be good in the long term.
And when measuring in the short run, there are other factors to consider: injuries, schedule differences, etc.
There is no easy way to measure it. But since the bills/browns share so many similarities (more than other teams), I think after next season it wouldn't be hard to ballpark which team is more improved. Especially if there is a big gap in the end results.