6 months, either in the darkest depths of the ocean in a submarine, or in upper Earth orbit in the ISS.
Which, why/why both fucking suck?
- Cramped conditions
- Instant death on the other side of the walls
- Limited human contact (AFAIK both have some connection for the internet so shit-sub/space-posting on 4chan would be possible but limited).
- Living without your usual comforts (living space aside, no TV, not much storage space for books, clothes, restricted diets, can't go for a run or get "fresh air")
- You need to be highly trained to operate or be assigned to them (for this scenario just assume you know how to run a sub or a spacestation, or all that would be sorted by everyone else on board).
- Besides this 6 month restriction, it takes a while to get back to civilisation, up or down, and across a bit.
- Being surrounded by highly compressed water (not that water is compressible, but that only means a rupture would get that pressure transferred to you and force you to the size of a peanut) vs being surrounded by a vacuum strong enough to boil your blood and burst your lungs. Neither pretty.
- You get a pretty view in space. Ocean floors are virtually deserts apart from the scattered wrecks and "islands" of life around carcasses and volcanic vents, and I'm not sure conventional subs have windows. Periscope?
- Flying at thousands of kilometres an hour vs I dunno, a few knots underwater.
- Following and creating a bit of a juxtoposition from my last point, in space you can't go anywhere because it's so vast, even if you weren't going around in orbit. In a sub you can "go" places that can be interesting.
- Experiencing virtual Zero-g vs the ol' 9.8m/s^2 everyday, and all the problems associated (or not) in dealing with that.
- No sun can penetrate deep underwater to the point it's pitch black and you'll return a vitamin D deficient pale fuck, vs too much exposure that you have an increased risk of cancer when you get back. Nice.
So, what'll it be?