The consideration for 1st or 2nd seating is what is best for your family...especially for the kids. For us where we had teenagers, the second seating has been best (2 different cruises).
As for the benefits, it is a bit more relaxed as a result of the servers not having to prepare for another meal that is just following. There were times that we did not leave the dining area until 11:00-11:30 at night. We just hung out with our tables mates, talked and relaxed. The servers worked around us and were always willing to give us what we wanted. My girls would take off and do whatever they wanted right after dinner and desert.