Would you rather live in a shitty life, knowing that it DOES get better 20 years later, or live a moderately ok life with no knowledge of the future?

Moderately OK.

The "get better" option is too vague. Better than shitty could still be worse than "moderately OK". Depending on how shitty the life, getting better could also mean death. Even if getting better meant a glorious life, there is no guarantee of how long I would get to enjoy that better life. I could die the same day.

Definitely depends on the details/levels of the shittiness and betterness. Is it like "you'll work really hard and be tired for pretty much all 20 years, but are guaranteed that part of "better" is being healthy and wealthy enough to deeply enjoy the next 30, guaranteed"? Or is it like real life, and you could die during year 21 after things have gotten 1% better?

Do I do the joke of "hurr already had the shitty 20!" or not?

When does the shitty start? Like, now? Because right now I'll take moderately okay. I'm already falling the fuck apart man and I know how good I have it relatively. Let's not tempt fate.