We did both.- paying corkage and buying a the Commodore Collection. The corkage fee recently on QM2 was $25 and a few cents and no additional service charge. (even the Sommelier didn’t know what the few cents represented).
The Commodore Collection was available to us at a useful discount before sailing but involved a phone call to Cunard. The CC makes a significant difference to wine list prices making the wisdom of paying.a corkage charge debatable.
it’s not difficult to do the maths and the decision really depends on your personal love of particular wines. Be advised though particularly wines in the CC do get replaced.