On the Fantasy last year, the early time was booked up, but we got in off the wait list.
It really depends on your normal dining habits, but for us... while the early seating is a little bit too early, the late seating was much much too late. We had kids 7-9 at the time, so basically eating at 9pm with young kids seemed like a non-option to us. If the kids are a bit older, it might be more of an option. But personally, I don't want to be eating that late.
Depending on your cruise and port schedule, I suppose it could run into your port time a bit. Our next cruise is Alaska, and 2 of our ports are until 7:45 pm. So I suppose it will end up being a choice between getting back a couple hours early, or missing the formal dinner.
Though remember you have options.... First off, the restaurant will seat you even if you show up late for your "early" seating. Though our dining time was 5:45, they seated us with no complaint when we arrived at 6:30. Secondly, missing a formal dinner or 2 is not the end of the world. If you can't make the regular dinner time, it could be a good evening to try 1 of the adult-only restaurants (Remy or Palo).
So to me, the early dinner time gives you more flexibility. You can eat early, and still have "night life" afterwards. Kids won't get whiny and starving.
I do think the very late seating is best for couples, or adults with teens who might enjoy eating late.