Back when I was a kid, it was always Nintendo for me. The NES and SNES defined my gaming childhood, and whilst I liked my Mega Drive, it was the SNES id play almost every day. Get since the PS1 came out, I've definitely been a Sony guy. I always have at least two of the consoles available, but whilst I played my PS1, PS2 and PS3 daily, my N64, GameCube and Wii only got used once every six months or so. Now I have a PS4, and don't think I'll bother with a Wii U.
The reason is depth and diversity, largely. Sony provide far more exclusive games than both of their rivals, and those games cover a much wider range of genres and IPs. Nintendo make great games, but I don't NEED another Donkey Kong game that played just like the Wii one, which played just like the SNES ones. Nor do I need more Mario games that played just like the other ones. What I DO need is games like The Order 1886, Uncharted 4, Infamous Second Son and whatever E3 throws at us; these are games with new stories, new worlds, new characters, and that's what excites me. Halo, Fable, Gears and Forza are all great franchises, but they're pretty much the only games that Microsoft put out, and that bores the hell out of me, it's the same with Nintendo. Sony strike the right balance between old and new; the PS3 saw the birth of Uncharted, inFamous, The Last of Us and LittleBigPlanet, while they continued fan favourite franchise like Ratchet and Clank, God of War, Killzone, Sly Cooper and Gran Turismo. That's what nails it for me; provide the games I love, while adding new ones on top. Constantly hammering Mario, Zelda, Donkey Kong and Metroid isn't going to do it for me.
Throw in the best third-party support, the best Japanese support as well as a genuine dedication to providing what the core fanbase want, rather than chasing the casual media dream, and Sony are easily the company whose intentions best meet my expectations.