I have travelled with SAS and Widerøe several times the last month (I counted I have had 30 safety demonstrations, and this is written Mid-February). Both companies are nice to travel with. Widerøe has their smaller planes, and usually, there is a free seat next to you, although not always. SAS has been filled to the brim almost every single time, so regarding space and well-being as a passenger, I prefer Widerøe.
The service in SAS is somewhat better, as they give you free coffee or tea, compared to Widerøe's mini chocolate. Widerøe have snacks for sale, SAS has a couple of food items, but are almost as limited as Widerøe, despite they have 100 more passengers in the airplane.
A final difference, at SAS you can pre-pay for your preferred seat, or you're offered a random one – unless you manage to choose your favourite seat out of those being available 22 hours before departure. Widerøe has free seating, so if you're early in the queue, you can take any seat, whatever you prefer, although the two seats facing backwards are only to be used if all other seats are occupied. Else, if there is very few passengers, they prefer that you don't sit in the very front of the airplane, it seems.