OP: that's a tough decision...I also wish I could give you the $64,000 answer but it does boil down to you having to do what is best with you and your family.
I prefer to work 8s. For full-time, 4 8s is enough for me--I'll take the 32 hours and the smaller paycheck than being forced to work 40. Also, working 32 allows me to easily pick up that 5th shift if I felt like it.
I used to work 12s. They're all right but they really eat up the day: after I've worked 12 hours, travelled to and from work, and made dinner. I'd find myself with little time to get anything else done before I had to go to bed.
I've done 16s a couple of times. Once was to get Christmas day off--I swapped shifts with someone who needed that particular day off more than she needed Christmas. The other occurred when they were having problems finding a nurse for the next shift and I told them that while I didn't prefer to stay if it came down to it I would...of course, staffing read that statement as "she's volunteered to stay, cancel the search."
While I can do them if I have to, 16s are hard on me.