Rainbow six siege, cuz better graphics, gameplay, teamplay, etc ¯\_(ツ)_/¯
R6 bcuz its better
CSGO imo is better. The game mechanics feel better and the game is in a much better state than siege. If you're more about IQ and smart plays, you should play siege. If you're more into aim mechanics, making highlights and strategy, play CS
In my opinion, I like siege better but csgo is better if you need faster rounds.