It's an impossible decision. Let's say Braxton decides to play the "Percy Harvin" position, and do some creative things at something other than QB, and JT Barrett develops a post-surgical infection in his ankle, and misses the whole year. Cardale would have a full year playing QB, and would likely be in position to earn first round money after the end of next season if he performed well.
On the other hand, if he is in a 3-way competion for QB next year, and he ends up sitting for most of the year, that wouldn't be good for him.
Right now, his stock is high, but has reasonable questions that go with it. What's the best he could hope for? 3rd round? But it could go to the 4-6 rounds. Then he is a late round pick with a decent signing bonus and a salary of 500K. Not bad, but far from a multi-million dollar signing bonus and salary.
Good luck to him making that decision. He could be costing himself huge money either way. On the upside, the consequences of making the wrong decision will still bring him more money than he ever dreamed of.