I don't think any of us could make a real recommendation without a lot more information about your situation (not that I expect you to provide that level of detail to us.) So I will tell you about my situation and you can see if it is anywhere close to yours.
In my case, I have a job that I really like (I don't quite love it, but I definitely don't hate it). I feel stressed because I feel like I am running around a lot of the time, not enough time with the kids, etc. We are not "rich" but we are comfortable. We can pay our bills, we are saving for the future, we still have some discretionary spending money.
If I could work 50% of my current schedule for 50% of the pay, I would jump at the opportunity. It would make me happier and we could do without the extra money. (I have been able to work 3 days a week instead of full-time for a couple of summers -- and it was wonderful -- but it's not available to me on a long-term basis at this job.)
I could be a freelancer and would probably make more per hour, could sometimes adjust my schedule so I wouldn't be as harried, etc. However, the work load would be unpredictable and I would have to take on more job aspects that I don't like (like accounting, blech!). I have not chosen to freelance because I think the unpredictability would really mean that I wouldn't be "happier," I'd just be trading one set of stressors for another.
If I had the opportunity to work part-time for more money at a job that I *hated*, then I'm not sure it would be worth it to me either. Again, I'd get more time/money, but the other stressors would cancel that out.
There's got to be a "sweet spot" where you minimize the stressors. If you're just trading one set for another, then it doesn't seem like a move "up."
So I think you just need to weigh your situation. If I was living paycheck-to-paycheck, I would be too stressed to be "happy." I need stability, I think. However, if I've got those things taken care of, job satisfaction/time with family, etc would make me a lot "happier" than more money.