If you have to buy only one, buy a PS4 : more games, more variety, more exclusive games in the long run, VR is coming, and the share play is fantastic.
Of course I would buy both if Halo is a game you really want.
PS4 is for the Gamers.
”Every great dream begins with a dreamer. Always remember, you have within you the strength, the patience, and the passion to reach for the stars to change the world.”
Harriet Tubman.