I think training both C and PF is what I would do. If you decide to train 2 positions, you can buy a 5th trainee as well. With a little practice, for 2 positions you should be able to set your lineups so that 5 players get at least 48 minutes each week (needed for full training).
As those trainees improve, it is a great idea to sell some of them to raise money, and then buy upgrades for the positions that you aren't training (as well as new trainees to replace the ones that you sell). I think that is a solid plan for long term development.