If you think you'll be a partner, and a good one, and it's what you really want to do, then 1.
Otherwise, 2-- and use the extra hours to stay alert and look for alternatives.
This assuming that the jobs are equally good in terms of colleagues, content, etc. And is Option 2 some incredibly expendable part of the firm that's offering it?
Option 1 is a big company; option 2 is in-house work for a well funded not for profit organization (not expendable).
for me this is a no brainer for #2. You don't have to put on your resume that it was 30 hrs/week, and you get 3 day weekends every week. I'd love a schedule like that. I get 3 day weekends every other week, but it still works out to 80 hours per two weeks.