I would use foam board- the reflective foil stuff does not last. A foil-faced foam board is available. Celotex is one brand.
I think your bigger problem will be to keep the moisture from condensing on the plywood and rotting it out. Be sure to install a plastic vapor barrier and/or foiltape all joints. Use expanding urethane foam in any spots where this can't be done.

I don't have an opinion (first time in 58 years) about which type of insulation to use, but I cannot emphasize enough the importance of leaving an air space and providing ventilation between the insulation envelope and the sheathing. I've designed and built homes since 1978 and that is one of the most important lessons I've learned. Failure to do so will result in destruction of your roof within a few short years!

Regards,

Mark