Gladiator is a little easier to play, and a little easier to heal. Marauder is a little more involved and a bit more panic-mode for healers, but IMO, more fun. Currently, Warrior (Marauder) is a bit underpowered at the very top end of end game, and that's done some damage to the reputation, so Paladins (Gladiators) are a bit more desired. That's only at the very high end, though, and patch 2.1 (sometime before the end of the year) is supposed to buff Warriors to balance them.

Whichever one you pick, you'll end up leveling the other one at some point anyway. GLA to 15 is required for Warrior, and MRD has some cross-class skills that GLA/PLD tend to use.