In an MMA fight? McGregor obviously. Im going to assume that that's not the debate.
In a street fight, McGregor is obviously more trained but 50 has the size and strength on him. McGregor could try to take him down but 50 might put him over his shoulders. Anybody can get their ass whooped in a street fight and 50 has about a 60 pound advantage on him. That is a lot. 50 might not be a professional, but the guy is in shape and with his boxing experience, he definitely has a lot more punching power than him. A lot of people overestimate the striking power of MMA fighters, especially when it comes to fighters in McGregor's weight class. I'm not doubting that McGregor can hit harder, but 50 might be able to hit him harder.
ONCE AGAIN, I'm talking about a random fight-encounter in the streets where they may end up relying more on adrenaline than technique and strategy.