I would further research the summer photos and reviews ( Trip Advisor, etc) for both options, the Sognefjord or the Hardangerfjord, since you can do this as a Bergen round trip your second day. If you really like trains the vintage Flamsbana train was pretty- you see a steep partially forested gorge and some large waterfalls. I thought the Sognefjord, for most of the journey, was very wide and more like a river, IF you need to use that 5 hour "express " boat it is a big chunk of your day.. This route seems to have more variety:
But I didn't see any of it, so can't compare!
That said, travel is SO subjective, I don't in the end trust anyone else's opinion! It's just useful for information gathering, so I hope this help a little- you will love Norway!