I've flown EWR-OSL with both SK and UA in Economy, and SK would be my choice, unless you get Economy Plus on UA. The food is better on SK + I prefer widebody as opposed to 757. You could also try to upgrade on SK via Optiontown.com, and Economy Extra on SK is in a different league than i.e. Economy Plus.
Make sure you visit both Bergen and Tromso, both fantastic cities.