Without kids, I definitely preferred the second seating. With main seating, I always felt more rushed to get back in from the beach/pool to shower, clean up, rest etc before dinner. A couple of times we even had to return from the port earlier than required so that we could make it to dinner (I know, we could have eaten somewhere else, but we liked main dining room for dinner).
With the second seating, we can come back in and relax for awhile before dinner. We can take our time getting ready and even go have a drink in one of the lounges.
Now that we have kids though, it's all about main dining for me. I don't want them up that late.
Only negative for me about second seating is that I didn't go to as many of the shows, since they were all presented before dinner. The whole point was for me to take my time getting ready, so to rush out for a show would have defeated the purpose of second seating.