I would rather be on the ISS, I think. The ISS has (tiny) windows with great views and more room per person (about 150m³ per astronaut).

I also think its inhabitants have more of a sense of being blessed to be allowed to be there. That helps, psychologically.

ISS is physically more demanding.

Radiation damage, weightlessness, constipation etc.

At least in the ISS, the astronauts stay connected to the rest of the world, unlike the submariners on ballistic missile subs.

An interesting thought, how many submariners have also become astronauts? Seems translatable to me.