I still think I prefer RDR, precisely because of the setting/story bits you've mentioned. It felt so unique to ride horses, to travel to locations that felt so different- snowy Tall Trees, to dusty bits of Casa Madrugada and to use Dead Eye targeting. Also, the wildlife aspect felt so unique at the time- those god damn cougars killing me at night by Fort Mercer! I liked the Rank/Challenges used to become a Legend of the West, and the dynamic events were awesome to experience the first couple times around. Camp fire saving, clearing out forts, all the mini-games- it felt like a world we hadn't been to yet.
That being said, GTA V's mission variety, shooting targeting, character switching feel like the the best systems ever employed in an open world game. I'm consistently astounded by the vastness of the game, the way different characters react to the world and the diversity of Los Santos. This game is truly wonderful.