Man you cannot go wrong with either. Do you want a compelling story with an insane amount of depth to the world? RD2 Do you want countless hours on multiplayer? GTAV
GTA 5 story was surprisingly enjoyable, multiplayer without a group of ppl to play with felt like it lost appeal very quickly for me. RDR2 looks fantastic but I have yet to pick it up, have heard v good things
get GTA 5 if you want to enjoy the massive open world setting with a lot of activity on it,
RDR2 if you want to experience some immersive world-building and great storytelling
It depends. If you think you'll play some online, get GTA V. I stopped bothering with RDR2 after I finished the story. The online content pales in comparison.
But if you want a compelling story game to play, I highly recommend getting Red Dead. It has one of the best protagonists in a Rockstar game and the world is so rich.