I can speak only generally on ships we have been on (not the Queens). It entirely depends on you! Do you like sitting for dinner at 8.30pm +? We prefer it. This will mean, for early-bedders, going to bed on a very full stomach! You can always stay up late at the disco, or at the post-dinner shows. it will also mean that you have a nice long relaxing time from coming back from a port destination before getting ready for dinner.
I'd agree with the other answer. I always prefer the second sitting. At some ports of call where you are not sailing until after 5pm, the first sitting seem to either have to rush to get ready for dinner or come back aboard earlier. Downside of second sitting though is that by the time you finish dinner and give it some time for your food to digest, you can be quite late to bed which does not always suit everyone, especially if the ship you are on docks early in the morning and you have booked on shore excursions that mean an early rise from be.