Ronaldo has finished first and second in the last two seasons, while Messi was runner-up last time out and third in 2007.
The duo are again likely to be among the favourites to win this year`s award, and Wednesday's game in Rome will see the two players go head-to-head in a match which could influence who is named European Footballer of the Year.
However, Messi said: ''I don`t see it like that. I think they are two great teams, who in my mind play the best football, and whoever wins the final in Rome will be proclaimed the best team in the world.''
Messi, who turns 22 next month, has already had previous dealings with United and Ronaldo after Barca were paired against the English giants in the semi-finals of last year's Champions League.
On that occasion Barca failed to breach United's defence as they lost 1-0 on aggregate, and Messi admits it was one of the lowest points of his career to date.
''On a footballing level it's the toughest memory,'' he told Marca. ''The truth is that it was a blow for everyone because it meant we would go two years without a trophy.''
It is a different story for Barca this season as they have already won the Primera Division and Copa del Rey following a remarkable first campaign in charge for new coach Pep Guardiola.
Now Barca are aiming to become the first Spanish team to win the treble of league, cup and European Cup, which Messi admits would put the icing on what has been his most impressive season since breaking onto the scene five years ago.
''It's been my best year, without doubt. For the football, the play, for the goals and I hope to crown it with the Champions League,'' he added in Sport.
''At the start it (the treble) didn't concern us much. We always took it step by step. Now we've won the league and cup we are going for it and we are going to enjoy this occasion. We want the cup, league and Champions League.''
Messi knows it will not be easy against the reigning champions though, adding: ''The football that we are playing could make this an unforgettable year, but ahead of us is going to be a very tough opponent who are also playing very well.
''Good football is not the sole reserve of Barca. I've seen Manchester United play and it seems to me that they are a very good team. I like how they play.''
Barca's hopes of being crowned champions of Europe for only the third time following their successes in 1992 and 2006 could be badly affected by injuries and suspensions, though.
Definitely out of the final are suspended full-backs Dani Alves and Eric Abidal, while their fellow defenders Rafael Marquez and Gabriel Milito are sidelined with injuries.
There are also question marks against Thierry Henry and Andres Iniesta.
Striker Henry, who has scored 25 goals in all competitions this season, has been sidelined since Barca's 6-2 win at Real Madrid at the beginning of the month, while influential midfielder Iniesta has been out with a thigh problem picked up a week later in the 3-3 draw against Villarreal.
Messi said: ''I hope that both can be in the final, they are very important and deserve to play in a game like this. It would be very good.
''Iniesta has had a spectacular season and it's the same with Henry, who has scored a lot of goals. It will be a lot easier if they are there.''