I'm still trying to unlock
but honestly and completely: Ground Zeroes was better paced with it's intro and outro. The major problem with Ground Zeroes was that the main mission was too short. However in tone and function, it is MILES ahead of Phantom Pain's "filler" and throwing everything into cassettes and never really having cutscenes until the back-half where it's apparent they were thrown together to try to salvage the project.
Compare Ground Zeroes "tone" in story and themes to Phantom Pains lack and you'll see where I'm coming from. Yeah, Phantom Pain has thousands of guns... if you want to go lethal... and options to go about doing that, but story-wise? It falls on it's face, and I'm one of those "just give me gameplay, stupid" people that complained about the numerous cutscenes in MGS4.
You're talking gameplay. I'm talking
story pacing and gameplay peppering through. There is some
serious pacing issues with Phantom Pain and the cassettes are just a lame way of fixing that.