I'd probably try to strike a balance between the two whenever possible. Conquering when absolutely needed and resorting to diplomacy when a more peaceful outcome could be reached.
A chill Viking who farms potatoes (yes I know potatoes came from the Americas, but this is a rad viking farmer village that already went to the Americas to get potatoes). Way ahead of the Irish.
I think I would be very diplomatic viking clan leader. Other side would always have a choice. If they are willing to be enslaved, they will not die, otherwise they will be conquered and mostly dead.