It really depends on your preferred eating times because unless you are on a very talkative table you'll make all the shows you want to whichever time you choose to eat. We prefer the later one because we hate to be rushed when we have been out for the day and like a more leisurely approach to meals. Others, I know, feel that by eating early they have the whole of the evening in front of them to enjoy without interruption. I will say this that it is invariably the early diners who go to midnight buffets, the rest of us are to full from late dining. There is a lot to do on QM2 and it should not just be the shows where you will find entertainment. I haven't looked up your itinerary but good weather is around in November in some unlikely spots so don't rule it out. Anyway whichever you eventually choose I'm sure you'll enjoy it. ... Ida.
All shows on ships are timed so that both dinner sittings have the option to see the shows. Have the people who told you this actually been on a cruise ship?
The 2nd theatre show is 10.30 and you will have plenty time to enjoy your meal on the late sitting before heading to the theatre. We generally left the Britannia restaurant at 10.15/20 but could have left earlier if we were not chatting to our dinner companions. It is only the theatre that is timed, other entertainment is on going. Whilst I do not agree with it, if the theatre is not full they will let latecomers in. My word of warning on the theatre is if you want a drink in there (bar service exists before the show starts for the low floor) take it with you as the service is very slow.
On cruises with ports of call, I prefer late dining so there is plenty of time to shower and change before diner, after a day ashore. On a QM2 Trans of course, this does not apply.