Well...you got the parts about their defenses right, but aggron has more attack. if it takes you five turns to defeat an opponent then they have five turns to defeat you. aggron has a base attack stat of 110 and bastiodon has a base attack stat of 52, more than half of aggrons.
BUT bastiodon has extremely high sp.defense, much higher than aggrons....they seem pretty balanced, but bastiodon is muchmuchmuch better at defense/sp.defense, while aggron is a better attacker.
And sorry I rambled without truly answering your question so here I go. I would go for more balanced defenses, because you could end up fighting someone that uses sp.attacks. and losing.
the answer to ur question is bastiodon