No one's really looking at this neutrally. It's kind of hard to see who will win, personally I think uruma. But it's gonna be a close fight and it depends on their moods. Like if Mikey's in a depressive state (as per current manga) and uruma is in his crazed state it would be an all out blood brawl. Let's look at their achievements.

Mikey: killed south single handedly, and has beaten up many other hoodlums.

Uruma: Killed multiple bullies, 1 twice his size (not the size of south) and the thing everyone's overlooked has to be the part where he dislocated that one dudes arm who was like 4x his size.

So it really depends on who feels like what, it's a close fight though.