Just as a lot of people have said, the two games are both great, but different from each other. Fallout gives you the chance to really make your own story, with a character that has very personal value. Even though Fallout 4, in my opinion, removed some key RPG elements, it's still very much an RPG. Metro is a very linear, very straight forward FPS, with a little customization options. I do love it's story, and it's ambiance, it's world, but it honestly has almost no replay value for me personally. The Fallout series has always been my favorite, and even though 4 kinda seems like a step back, I still love it. The only post apocalyptic game I've played that feels kinda close to what Fallout delivers, over all, is the STALKER series.
"they asked me how well I understood theoretical physics. I said I had a theoretical degree in physics. They said welcome aboard." - Fantastic