Louis Tomlinson -- Sorry Messi ... RONALDO IS BETTER!!!

Say what you will about Louis Tomlinson, but the One Direction singer's got a serious pair of cojones for taking a stand on one of the biggest arguments in sports -- Messi vs. Ronaldo.

Louis -- who dabbles in pro soccer himself -- was leaving a tattoo shop in L.A. this weekend when we asked him ... who's the better player??! The pretty boy or the Atomic Flea?!

It's a raging controversy in the football world ... here are the cliff notes for the debate:

-- Ronaldo has 38 goals in '13/'14 play ... Messi has 27.
-- Messi outscored Ronaldo by 18 GOALS in the 2 seasons before that.
-- Messi has 13 more assists than Ronaldo over the past 3 seasons.

But ... Ronaldo is bigger where it counts -- his wallet.

According to Goal.com, Ronaldo's worth €148 million ... while Messi's only worth a puny €146 million.