I’ve cruised with both lines.
Oceania is far superior in service and food than NCL but NCL’s food and service is fine. Oceania’s entertainment is low-key whereas NCL’s is more up beat.
We love Oceania’s small ship experience and that All non alcoholic drinks are free eg latte’s, fizzy juice, non alcoholic cocktails, smoothies etc. Oceania is a very polished operation whereas NCL’s can be a bit haphazard.
Oceania’s tours will be more expensive than NCL but most ports in the U.K. are doable on your own or by using local tours.
Only you can decide which will suit you and work out which, for you, is the best value.