My own two cents is that between Conjurer and Gladiator, it doesn't matter too much when you do it (outside of making sure to get your requirements for Paladin).

However, if you would like to increase your versatility and level Marauder, I would recommend leveling it alongside your gladiator. They literally use all the exact gear, minus the weapon, so gearing up is far simpler than if you were to level up one, then the other. The same applies to both casters, both healers (though SCH is only 30+), the three physical DPS (only up to 30-ish, when you start getting chainmail that caters to lancer, and gear that doesn't have both STR and DEX on it to share between archer and pugilist), the three disciplines of land, and the eight disciplines of hand.