I have worked both..I am soon to return to 12 hour shifts by choice. I have found that working a M-F or even M- Thur 8 hour days are really 9-10 hours & I am more tired, more stressed than I was working the 12hours. I also like having some flexibility with the days I work w/the 12s versus having to work appointments around on days I worked 8 hours or using up the Friday off for those things.
Not having children I'm sure makes a difference as I don't have to be concerned about getting home after a 12 hour shift to get homework done or missing a ball game.
I would be hesitant to work a management position OR an hour 8 type job again..for me, neither are worth the headache that comes w/them (I know you didn't mention management, I'm just commenting on that as well). I am looking forward to going in, doing my job, then not thinking about it again until the next work day.