I gambled on MGS V, being new to the franchise, and I honestly didn't enjoy it or play it for long.
If you've never played Metal Gear before I would just suggest you research the gameplay first.
Just Cause 3 though I can confirm is great. An improvement on JC2.
Much more beautiful, fun to travel across, densely populated, with a better story (although story still takes a backseat).
If you were to play it safe then you wouldn't be disappointed by choosing JC3.