I've done 2 transats on QM2, once in Princess Grill (think J class) and once in Queens Grill (F class). Prefer the westbound leg ex SOU, as already stated above, you gain an extra hour each night.
You can also travel Britannia grade (Y) and Britannia Grill (M+). It's a really lovely way to travel. PG and QG guests have an exclusive lounge and outside deck area, butler service in QG suites too. Try it!!