Either dress length will do which one you like most? and DJ or lounge suite are ok and they are probably less formal than Cunard and you may find that the dress is not strictly enforced .try the Celebrity web site and find the dress code it will give you a clearer idea and they both use the same dress code?
'Cocktail dresses' means short dresses but long dresses would be acceptable as well ...alot of women wear evening trousers as well with a glittery top that's quite acceptable! My partner usually wears a dark suit with white shirt and bow tie rather than DJ but most types of jacket are acceptable (I think the most important part is the shirt and tie rather than open necked shirt) On each ship we've been on there has been a mixture of outfits for formal nights and anything goes within reason!!