Well i'd say it depends, do you only have enough points to max only one of them out? Or can you have a little bit of both?
If you can get a little bit of both, I'd say max Fort and keep mdef at 3 since it always gave me enough time to do w/e with 16 seconds.
If you absolutely only want one of the skills (to say the truth.. kinda hard to decide), but i'd say keep mdef since usually with competent healers, they wont die to often/easily and if your particuraly good at dodging monster attacks, u'd be able to survive without Prot. Fort, and with only pots/playdead/counter, etc.