As everyone has already said, Conor in the UFC and Floyd in the ring obviously.
It's honestly a win win situation for both fighters. Floyd's ship is sinking in terms of popularity especially after leaving a sour taste in everyone's mouth with Pac/Berto. This would be THE fight to put him back on the map. He doesn't really need it but if he's looking for one last great payday, this is it. If it's held under boxing rules, even more so.
However, I don't see the UFC ever allowing the fight under boxing rules. Protecting its fighters and especially their golden goose in Conor from possible embarrassment should be their number one concern. I could see it happening under some hybrid no grappling allowed but kicks allowed type rules under the UFC banner though. Conor has absolutely nothing to lose really. A loss to one of the greatest boxers of all time won't hurt his cred much unless he is absolutely humiliated.
$100M for one fight is most likely more than he'll ever see in his career. That is absolute peanuts from what the buys this fight could generate and possibly even come close to the Pac fight. Not to mention that they are both excellent trash talkers along with it being the first cross promotion/sport fight, and the whole Floyd coming out of retirement angle and fighting the hottest fighter in the biz at 40 years of age.
I want to see it but I wouldn't just mind Conor shelling out L's at 155 either.