From memory, either side is good. The train changes side (of valleys, fjords & rivers) a number of times so each side has great views at different times. Plus, it’s a long journey, so take the opportunity to get up, walk around, check the views from the carriage ends, have something to eat in the dining car. This will give you the opportunity to see all around.
I traveled between Bergen and Voss (on the road) recently and drove alongside the track a lot of the way. Leaving Bergen you enter a tunnel pretty quickly and then pop out at Arna. The train then proceeds along the right side of the fjord for a good hour or so, meaning that if you want to see fjords sit on the left, if you want to see mountains up close sit on the right.
However, as above, from memory once you hit the uplands it’s all pretty spectacular on both sides.