Plaza for proximity to hookers and drugs, Scandinavia is in a far more civilised neighbourhood. Depends what kind of stay, it sounds like leisure and I'd go for the Scandinavia then. Nice location which you can walk to city centre, Aker Brygge and the west side of town. Scandinavia has a better feel to it I think, compared to the great glass tower that is Plaza. Make sure you have drink or three in the Summit 21 bar in the Scandinavia hotel for great views of Oslo and the fjord, especially at sunset. Use the toilets when in the bar, you will enjoy it.