CJ's value to the Bills may have increased exponentially with the OL seemingly fixed. He may well have his career season if kept around. The flip side, of course, is that as a free agent he may well be gone after this season.
The Bills game plan clearly is "the future is now." That being the case, you probably keep CJ. Freddie's trade value is minimal because of his age. But if someone is in dire need of a back who can likely come in and produce immediately, age notwithstanding, trade Freddie and add more draft picks for next year (love to get that number 1 back and I'm sure Whaley is working on it).