Hmm, in my headcanon they do have a clear preference for Reigen domming later in life, wheras I'm actually quite open for top/bottom switching & think they could both enjoy it both ways, even though they're both much more used to Reigen topping. I'm just extremely picky on how MobRei happens.

Tbh, it's hard to say what MobRei I like most because of that, it's honestly much easier to say it by comparing it to what I can't have... orz
But I'll try.

Most importantly, Reigen needs to still lead in a way. I said it before, I don't recognize them as the ship I fell in love with when the dynamic is completely different. I need Reigen to give Mob safety, no matter who tops or bottoms. (Which does not mean it can't be mutual!)
Then I need them to both want it, but especially Reigen, which is why I prefer Reigen initiating. Though I do think it can be kinda cute & funny if Mob just really wants to top, but for me it won't happen until Reigen allows it to.
My favorite top!Mob is obedient, eager to please & working to be praised. Maybe accidentally rough sometimes without even knowing it when he gets a little too excited, but never out of ill will. And he's all ears & loves to learn from master. So - a dog, all around. He's a puppy top to me.

Mob is kind & sweet & by the end of the series he is also better at expressing that & at allowing himself to be happy. So for anything post-canon, which MobRei always has to be for me, I need to be able to see that in Mob.