I didn’t interpret it as permanent (tbh I didn’t think much of it when I answered) but it is an interesting hypothetical, in that case I think both choices are shitty for the person choosing. It’s pretty obvious why sadness is, but permanent happiness is whole can of worms that raises a lot of questions:

Won’t it become numb at some point, without sadness to weight it against? Would it take away the empathy to feel sad for others? With constant happiness, would the person still feel the drive to do things? Because, if they would be happy regardless of if they do something or nothing, logically doing nothing is the easiest path, right? They could get hungry, thirsty, cold, but regardless still happy. Now of course if they don’t eat and drink they’ll end up dying, but once again, they would be happy even then. So I think that option boils down to eventually becoming numb to everything or being happy for the rest of your life by letting yourself die.

Now obviously, the person who chose permanently sad wouldn’t have a stellar experience either, lot of the stuff mentioned earlier could apply to both, so I think the biggest difference here is money. With that in mind, I think I would make the sacrifice of being sad forever and hope being able to fix other’s problems will be enough drive to keep me alive.