They are both amazing games, but I think it's not particularly fair to judge which is the 'better' one of the two given how completely different they are; both in terms of the genre of game it is, and, of course, when these games were made. It's easy to turn around and say Red Dead 2 is better due to the graphics, animations, overall mechanics of the game, etc., and this may be so, but we've got the remember that the technology has improved vastly since GTA V came out.
GTA V was revolutionary back in 2013. It was trying things that open-world games simply didn't, or couldn't, experiment with at the time. Plus it's number of sales made it an economic powerhouse.
Personally, I'm more of a Grand Theft Auto fan than I am of the Red Dead series, but that's not to say I don't like Red Dead; I bloody love it. Red Dead 2 is the single greatest story/campaign game I have ever played, and I will never forget completing it and feeling a huge mix of emotions. I got the same feeling with GTA V, but for some reason I felt more attached to Arthur than I did to any of the GTA V main characters.
It's a tough one, and it's mainly down to personal preference. I just cannot wait for GTA 6.