I've done early with a four and six year old last year and late with a six and seven year old just this summer. It was our first time trying late seating and I felt it had pros and cons. I LOVED the extra time it gave to every day. Early seating pretty much eats up your whole early evening from 5:30-7:30 while the early show is a lot quicker. There were a lot of great activities going on in the kids clubs that they would have missed otherwise. The dining room was also a bit quieter and less frantic during second seating.
On the other hand, one huge disadvantage of Disney is that there are so few options for eating between 2 and 6 pm. There's crappy pizza, greasy burgers, hot dogs and fries, a few boring ice cream flavours and maybe a couple of wrap and sandwich options. Having cruised on other lines with better dining options I was really disappointed. There's just no way my kids can go from lunch until 8pm without food and those options just leave us feeling bloated and gross. We did room service a couple of days, although one day we had to wait over an hour because I guess everyone else wants some real food at that time too. Another day or two I made a run up to the buffet just before it closed to get some cheese, breads, fruits and veggies and kept in the fridge until 4 or 5 so I'd have something to offer when they came back from the kids clubs starving. It's never been an issue for us before since we've only done early dining, but I was really disappointed with their options for those of us waiting for late dinner with hungry kids.
Like some others here, my kids found late dining to be just too late for them. They weren't falling asleep at the table or anything, but they were more bored and restless than they usually at early dining which is usually a sure sign that they're over tired. They also didn't eat well that late. I usually gave them a large snack/small meal at about 4-5 pm and they seemed hungry again by 8 but they would just pick at their food most days. It just felt eating that late threw everything off for them. But of course, every child is different.
One suggestion if you're still on the fence is take a long weekend or something when you can stay up late and sleep in and try serving your family supper at 9pm every night and see how it goes. Maybe it will be a fun change. Maybe your family will wonder why you're torturing them. And that might be your answer.