We did NiN from Oslo to Bergen in March.
On the Flamsbana from Myrdal to Flam the left side might be slightly preferable but no-one stayed in their seats. People just got up if they wanted to take photos. There are no reserved seats on this train. The windows open so you can get good photos.
On the bus from Gudvangen to Voss it didn't matter which side as the road is windy and the view is mostly mountains. From memory we sat on the left immediately behind the driver. As I get bus sick we try to sit where I can see out of the windscreen.
On the train from Voss to Bergen we sat on the left. I don't think it would have mattered which side we were on as our view was mostly snow. The windows are very large (but quite dirty) so most of my photos look blurry.