It depends whether you want a bulky staller, or a strong attacker.
Eviolite Lairon pros: Bulkier, easier to take super effective moves, less expected
Eviolite Lairon cons: Can't attack very well, knock off, No item freedom,No chance to outspeed anything
Aggron pros: You can run all types of different sets, item freedom, good attack, can still take one good hit (unless it's ground or sand), better speed.
Aggron cons: Less bulky
It depends what you want on your team; all in all, though, Aggron is stronger.