Ive played around with Gladiator during P4 and this is the easiest conclusion one can do toward these two tanking class;

1. Gladiator > Paladin is the main tank, normally use to tank boss due to their higher damage mitigation (evasion/parry/block). Gladiator lack AOE skill to tank many at the same time so Gladiator is easily the single target tank. They have lower HP compare to Marauder but higher damage reduction/mitigation.

2. Marauder > Warrior is the adds tank, normally use to tank bosses spawn and adds. due to their higher HP. Marauder only mitigation is evasion and parry (since they lack shield to block) but they have higher HP (by a large margin) compare to Gladiator. Marauder have alot of AOE skill so Marauder can easily gather enmity(threat/hate) around larger number of enemy.

So which are better? NONE. they work alongside each other in 8-man dungeon and 4-man dungeon can be done with either of those two. Marauder is harder to master due to the steep learning curve.