My Mum and Dad are big cruisers, hear about them a lot when I see them! They would equivalent of BAEC life time gold with Cunard (not that you get huge benefits with cruise loyalty programmes)
If you want to cross the Atlantic in the old fashion 'silver service' style, akin to the Titanic etc then only Cunard offer this. Whilst the other Cunard ships (Victoria, Elizabeth) are just off the peg ships. The QM2 is a bespoke ocean liner and the only one of its kind left. Not quite a streamlined as the QE2 but designed to take the north Atlantic in mid winter.