I'd recommend +MAG/-LCK, personally.
The reason being, +MAG/-RES has the following +/- Stats at Maximum Level:
MAG +3, SPD +1, RES -1
whereas +MAG/-LCK has these:
MAG +3, STR -1, LCK -3
You lose on STR with MAG/-LCK, which you won't need anyway as a Mage [it will help a bit with Ignis, but -1 STR won't be terribly noticeable], but you keep your +1 SPD [every little bit helps!] and don't lose any RES [which is definitely appreciated]. You do lose -3 LCK, but honestly, it's not as important a stat as the others [in my opinion].
Good rationale. My only hesitation toward reducing luck is that since I'm beginning on Hard/Classic, every enemy critical that I can keep from happening is a lot of time saved for me. I'll think over my options and go with something soon (I've been agonizing over this for an embarrassingly long time, heh)