I'm from the NW, too and can offer my opinion about the boat option. However, my trip was in July (5 years ago) and also Bergen was my destination, so, a 1 way journey.
I mention the season because there was barely any snow along the train part- it was lovely, but underwhelming to me, spoiled by our NW scenery.
I opted for the boat from Flam because I read about the steepness of the bus option and I am easily car sick (and boat sick! but afraid of heights) hopefully you will hear from another Forum contributor about that!
The beginning was beautiful, but if memory serves it was almost 4-5 hours on the boat? Getting into Bergen about 10 pm, but plenty of "day"light still. I remember the last couple of hours going very slowly, once the fjord became wider, and without much to see. But it would be relaxing to read, etc and be protected from the weather in the indoor area if needed.
I am not sure how much this helps, but hopefully at least a little! I hope you will have a fantastic trip regardless of what you decide!