The answer is obvious:
Ronaldo would do roughly what Lukaku does now, except that he's obviously a better goalscorer so would almost certainly score more goals. But given that United were pretty crap for two-thirds of last season, we don't really know how many chances he would get. We can say quite confidently that he wouldn't get anywhere near the number of chances that he gets for Real Madrid.
Whereas if you sign Messi, you now have the best passer in the world, the best dribbler in the world, the most creative player in the world, the player who broke the La Liga free-kick scoring record last season, and someone who has consistently scored 40-50 goals from the number 10 position, while also topping numerous other statistical categories.
Bear in mind as well that Messi has been Spanish player of the year for season after season. You can argue that Ronaldo has performed better in the Champions League (I don't necessarily accept this, as it's misleading to look at team performances and goals scored, and then draw a conclusion from this), but Messi has massively outperformed him in La Liga. If you sign Messi then you're getting a considerably more complete player, who has performed week in, week out at an incredible level for over a decade, in a way that no other player has ever matched in the history of football. And the league is obviously more important to United at this point in time, and indeed historically and culturally.
Whereas if you sign Ronaldo, you're getting an extremely good target man, who is nearly three years older.
Anyone who would choose Ronaldo over Messi either doesn't understand football, or is incapable of being objective for some reason.