GTA V is basically an expanded San Andreas map with more to do, better features, and more content. Plus better graphics and more choices. Whether you own an old PC or not, it's hard to argue in favor of San Andreas unless you like the storyline that much. Plus, GTA V has real multiplayer
GTA V is not an expanded San Andreas map. GTA V only features expanded Los Santos and the countryside and deserts around it, and that's about it. In San Andreas, there was San Fierro and Las Venturas. I loved both of these cities. And as far as multi-player goes, I prefer San Andreas' multi-player, even if it was just a mod. However, it came a long way since it's release a long time ago, and has improved a lot in terms of sync, connectivity and stuff like that. Why do I prefer it? GTA V's multi-player is just a bunch of missions you can do with or against your friends or other people, and the only thing that's good about it are the Heists, and once you beat them, there's not much to do. In San Andreas, there were tons of different game-modes and servers to choose from. You want a bit of deathmatch? Join a deathmatch server. Want to role-play? Join an RP server. I loved RPG servers. I loved the concept of getting a job, buying a car and a house and being one of the richest guys on the server.
Back to the question. I would definitely choose San Andreas, as you can see from above. My PC can run GTA V on high settings with a smooth FPS, but it's just not it. I have completed the game once, and couldn't be arsed to complete it again, whereas I completed San Andreas for at least three to four times. After completing, I played some multi-player which was fun at first, but not that much after a while, and the Heists are the only fun thing. I have played San Andreas ever since it released until 2009 actively, as in, pretty much every single day. After that, I play it once a year for a few days, just for my nostalgia to calm down.