I never said this approach works long term or generalizes to all situations. Broadly, I am going to take the better player period. For the purposes of this discussion, I'm referring to the specific comparison between Gurley and Bell.
The better player is pretty obviously Bell.
If you think Gurley is a better talent so be it. Talent rather clearly does not always translate to being a good player.
Based on scout ratings, the better player is not pretty obviously Bell, if we're talking about talent. Gurley consistently has received much higher ratings than Bell. The better talent is pretty obviously Gurley.
To clean this up a bit, I think we're both on the same page in saying we want the most productive player. It's sub-optimal to have a talented guy mired in an bad situation on a bad team. On the other extreme, it does no good to have a guy who stinks playing in a favorable situation. That said, history is pretty clear that the former is far superior to the latter. Think of guys like OJ, Dickerson, Sanders, even Faulk and AP on various iterations of the Colts and Vikings, respectively. Those guys were on bad teams, sometimes terrible teams, and produced at elite levels because their talents represented a variable that was independent of what the team or scheme around them supported.
Bell is not that guy. He's just not. He is going to need 80 receptions each season to maintain top-5 production, let alone enjoy a year like he had in 2014. And, if you want to bet on that surviving over the long haul, that's lost money. He is a good and decent running back, but not a top-5 talent.
Plug Gurley in anywhere, he is going to be a dynasty monster for years to come.