Just pick one. If you don't like it, level them evenly. There's plenty of XP to do so, and it's not like you're penalized for taking your time to enjoy the content.

You might find different aspects of each job where you enjoy it more.

For example, I'm a bit different because I'm by pretty much all counts a veteran in this game, BUT:

- For new stuff, I prefer healing (typically as SCH, but WHM on occasion), b/c it's what I'm good at and I can practically play my class blindfolded.
- For old dungeons (or even new after the first 2-3 times)...I prefer tanking (used to be almost exclusively as PLD, but I really like GNB, and the new DRK isn't bad to me either). I like setting the pace (which when I set the pace I strongly lean towards preferring to pull ALL THE THINGS), and I consider it less stressful than healing when I want to be mindless.
- For 8+ player stuff, I prefer to DPS...in particular, I prefer to be either SMN or RDM (so I can help rez if the healers fall behind or, worse, if they both die)

I tend to level SMN/SCH first, largely because that's what I'm most comfortable and familiar with, followed by a tank, and then everything else. I used to watch videos before running stuff as a tank when I first started, now I'm more likely to (outside of savage/EX) go in blind...but that's more that I've gotten much more comfortable tanking (and for the most part, comments I've gotten from other players makes me believe I'm decently good at it).

In your case, though, just play. I'd start out leveling them equally until at least lv30ish (until then, everything is a snoozefest no matter what you play anyhow)...and then just go with what feels right.