^ or to put in another way
1. Gladiator is versatile. he puts out great damage and can tank well -- he is all around, heavy damage, heavy tanking, and especially great for botting if you're going to do that.
2. Templar has THAT much more tanking than gladiator. Templar's power is tanking and has much less damage than gladiator. When it comes down to it, especially in end game Aion, Templar is the tank, not gladiator.
So if you want to be relied on in groups for tanking, be the one in the front lines pulling people in via PVP, and just want to be the ALPHA MALE of the group, go Templar. If you want to be powerful offensively all the while not being squishy, go gladiator.
Watch some Youtube videos of level 60 gladiator / templar pvp and what not, and see which you like better.
templar = shield/sword , gladiator = 2 handed spear