Armored bear = Magmite replacement?

Pet - Crystalline Magmite (90) - Armoured Bear (90)
HP - 2934 - 3149
P.Def - 8792 - 7618
M.Def - 5861 - 7618
Acc - 1354 - 1259
Eva - 1003 - 933
Speed - 6.2 - 8.6
Atk - 3240 - 1672
AtkSpd - 0.6 - 0.8
Dmg/s - 1944 - 1338

Pretty similar.

Given that while grinding, you'd probably only be healing your pet once (possibly twice?) per fight, I can't imagine the differences in defences would be too much of a problem. The golem might end the fight a swing or two earlier due to its high attack value, but if the bear gets a round in earlier due to its higher movement speed, they'd probably be pretty well-matched. The damage from skills will be higher on the Magmite however, due to the higher base ATK.

For bosses, the bear may arguably be better due to the higher HP and better-averaged defences, but of course for many bosses the dominant damage type is still physical, which would make the magmite beneficial. Fights would also be longer with the bear due to its lower damage output.

In all, I don't think there's really a clear superiority. Either one would make a fine pet.

EDIT: Forgot to mention, as far as aggro is concerned, unless you're relying on DPS, then the type of skills used and their level will be of more concern. Of course, with the bear so long as you're not pulling hate off the bear more often than once every 4-5 mobs, you can use Pounce to stun it and buy yourself time to kill it before it lands a hit/spell.