Use the Yak for IMAC. Bigger flies better and it is easier to control. Use the CAP580 for practice. Don't practice with the CAP just before a contest. Welcome to our nightmare. I would suggest the 20% for the 4 strokes. The YS does require a different tank set up. It is pressurized and some tanks will burst under the pressure. For simplicity use the OS.
EXCAP232