I would definitely pick the Scandinavia. The area is much nicer, close to the Castle and on the "right side of town."

What has happened during the last year on the backside of the Plaza is that gypsies have set up camp there and 50-80 gypsies sleep there every night. They also cook their dinners, go to the toilet etc etc there... its a little park with no facilities, so its really messy.. I really would avoid this area.

The area around the Plaza was never charming anyway imo, but very central and 1 minute walking from the central station. Hotel itself has decent elite recognition, usually upgrades to business class room and gives welcome amenity to Golds.

Ive never seen any prostitutes around the Plaza, but you will find some drug addicts, usually begging on the walking bridge to the central station.

The Scandinavia on the other hand is in a very nice location, feels much safer and is still very central. For tourist activities I would say it is a better location than Plaza. Rooms can be rather dated, elite recognition not very good. Honestly you wont get great elite recognition at any Oslo hotel anyway, so just pick the better location and enjoy the outdoors.

Oh, and I have stayed at the Park Inn once. Its a more basic hotel brand in the CC family, rather new and basic but good standard. Free breakfast for all but much less elaborate than the Radissons mentioned. Very central location as well, but I did notice a lot of prostitutes popping up in the evening in the surrounding streets, if that is bothersome. Zero elite recognition at this hotel.

Nydalen is a residential area a bit north in the city, I would not stay there as a tourist if there is not something particular you want in that area. Area is nice and quiet.

Enjoy Oslo!