It is definitely a personal preference. Some nurses really like the extra day off and are willing to work the longer day. Others find that the extra day off isn't really extra because they are tired and spend a day recovering. Some find the 8 hours better with a school schedule. Others find they can make their schedule such they can almost take a week off in between. One suggestion would be to start with 12s and see how you like it.
To answer this is all about your personal lifestyle and schedule. So for me a 9-5 schedule doesn't work, 4-8 hour days are not a thing for ER nursing, and with my husband's work schedule working 3 12s are the best, and you acclimate quickly to the 12s.
Personally 3-12 hour days. The extra 4 hours isn’t bad at all. Plus you get an extra day off during the week.