My gut instinct is to tell you to go with white mage. That's what I did after all when I decided on one job to take to 50. It's true that pretty much any party is going to need a white mage or two, there isn't a strategy out there where healing isn't a part of it. Hell, even on black mage burn strategies, I get called to apply stone's magic defense down, and I've even been called on to dispel crap with aerora in a few fights.

However, there is a downside to making WHM your only 50: White mage is NOT a strenuous job to play in this game, and so unlike other games out there, just about everyone has white mage levelled. So, it's still high demand, but it's also high supply. I still think it's your best bet, but don't breathe too easy once you have it at 50.