How cold does it get at night in December in Nong Khiaw? We're honeymooning, and so were going to splurge on the Riverside, but saw some recent negative reviews here and on TripAdvisor, and are now considering the Namhoun Guesthouse instead. Are the electric blankets at the former worth the $$$ + questionable service?
December nights can be pretty cold by SE Asian standards. Down to around 10C/50F or less at night at that moderate altitude (350m) which isn't all that cold unless you have no heating and a river breeze picks up. The Nong Kiau Riverside has electric blankets and hot water showers, but it doesn't do much good if the electricity cuts out which (as you seem aware) does happen now and then. I usually bring some thermal underwear top&bottoms for that time of year, and a fleece and wooly warm socks are welcome too. Daytimes are usually great though - sunny clear and mid-20C/70F good exploring weather. Anyway I would stay at the Riverside primarily for the views whereas the Namhoun sits in a garden away from the river.
Luang Prabang usually means a proper room with decent walls and sealed curtained window (depends on where you actually stay of course) whereas Nong Khiew Riverside is a more exposed bungalow setting. That said, I've been pretty cold walking around Luang Prabang in the evenings that time of year so a decent jacket is still a good idea. Also consider how much time you might spend outdoors after dark since NK has very little "nitelife" and LP has just a few bars mostly along the Nam Khan, but sitting outside the Hive Bar for a few evening beerlao's can be nice with a fleece. In-room heating is more or less absent from either of the two towns.