When To Use Mega Steelix Over Mega Aggron In Competitive Pokemon

I have been playing Pokemon Showdown for a while now (I still play in gen 7 OU, because I really don't like the new gen 8). Every time I want a defensive steel type, my go to is Mega Steelix. Why? It has a super high defense stat, access to curse-gyro ball, which lowers it's speed while raising it's attack, making the next gyro ball twice as strong. Also, curse raises it's defensive stat, and you don't need to panic about special defense because that sucks already. That's one of the problems I most often find, it's not a reliable sweeper because of it's special defense. But it doesn't stop there, Mega Steelix also has rest-talk, meaning it uses rest, go's to sleep at the cost of replenishing it's health, but then sleep talk allows you to move while sleeping.

Mega Steelix was working really well for me, until I found my friend at school always uses Mega Aggron. They're advantages to him too. His base stat total is 20 points higher, and it has higher attack. However, it's only good move is heavy slam, which is really good don't get me wrong, but curse-gyro ball is better. It does get rest-talk though.

So the question arises, when should I use Mega Steelix and when should I use Mega Aggron? I mean, what team structure is one better?