I would take 4 on 4 off with 12 hour shifts any day as I enjoyed having the days off. Back to the real world of non shift work for a bit now anyway!
I haven't had to do any flight planning as that was outsourced but did have to flight watch and as in most Ops department it was peaks and troughs, you would be busy for 2 hours then very quiet for the next 2 and so on. No cargo aircraft for me, all pax.
I ended up doing longer shifts sometimes when on 8 hours than on 12.
Have ended up many times having to do 11-13 hour days when it should have been 8 but then with the 12 hour shift I have rarely had to work much longer than 12.
But it depends on the set up of the Airline and the Ops department, where I worked 12 hour shifts as I have described were fine.