Well, like you, I've never played any MGS games either or any AC games on that matter.

MGS5 is the first I believe to go full open world and MGRR did have a pretty good storyline.

I feel you may need to play the previous games to understand the story or else you'll be confused as hell.

So I guess it's more of personal preference since one is based more on futuristic events and the other is based on historical.