A300-600F will be a a success for sure, because there are more A300 still in operation than A310. I think you should bring the A300 to the MFSF first and than the A310.
There is also a cockpit redesign for the A300-600 on the way. From my point of view, there are more arguments for the A300-600F with the old and new cockpit for the MSFS. I would buy it fore sure.
A300 aircraft in operation: more then 100
A310 in operation: ~ 25
Cheers