What is important in choosing a Ph.D. program? I'm now choosing a Ph.D. program between two schools and am struggling a lot. Please let me know your thoughts and I'll greatly appreciate any help! The two universities are very similar in reputation and compensation.

School 1's lab: The professor is very nice and supportive. He would support a broad range of research topics and I have the freedom of choice. The students are very hardworking. They published 8 top conference papers last year.
However, the students in this group are from not good universities. And the professor is very young, his oldest Ph.D. student hasn't graduated yet, so I don't know the students' outcome. Also, the students are so hard-working that the work-life balance might be bad. Also, the professor doesn't have as many connections in my desired place of work.
School 2's lab: The students in this lab are all from top universities. The professor has advised many Ph.D.'s and all of them are going to great labs and universities now. The professor has a lot more citations and more connections in the country I want to work in the future.
However, the group only published 3 top conference papers last year. The professor is also not as committed to personally advising me.
The students in this lab are all from top universities

it is an irrelevant criterion for choosing a Ph.D. program for your

... not as interesting

Unless you don't mind being miserable throughout your P.hD, you shouldn't work in this lab if you don't have to, which seems to be the case.

... the professor is very young

I think it is a plus, instead of a negative.

the students are so hard-working that the work-life balance might be bad.

Just because they don't know how to balance their work-life with their life outside of work, it doesn't mean you won't be able to too.