So you're planning to rent a car?
I don't consider the express boat back to Bergen part of the Norway-in-a-Nutshell route. From the reports I've read, most of its time is spent traversing a wide part of the fjord where you get views like you're on a river. It doesn't seem worth it to me, but of course I haven't taken the trip myself. Simply taking the train back to Bergen from Voss would be both faster and cheaper. The trip to Bergen is much faster than the trip to Oslo, but there's some good scenery from the Myrdal-Oslo train (much better than what you see between Voss and Bergen).
In addition to the Naeroyfjord ferry between Flam and Gudvangen, there are RIB trips on the fjord--I think from Flam--that some folks have really liked. I haven't taken one of those either; I'm not a speed junkie, and I think they're pricey.
I was very glad I managed to see the Borgund stave church, which would have been much easier if I'd had a car. As it was, the bus was very late departing from Flam so I really only had time to walk through the small stave-church museum; there was no time to read any of the explanatory material. There's a small cafe in the same building. I believe Rick vouches for its soups.
In an earlier thread about rural Norway, someone made what I think was a perceptive comment: Views are different from roads than from boats down on the fjords. I didn't see any of the elevated views except from the Flamsbana and the Gudvangen-Voss bus. It would have been nice to do some driving in the area, but I'd have been a basket case if I'd had to back up to let a bus get around me.
As for the practicality of staying in Voss, I have no experience with that (or with driving anywhere in Europe), so I can only suggest that you use ViaMichelin.com to get driving-time estimates and research the cross-fjord ferry(ies) to be sure they run frequently enough not to impede your plans. I know the express boat (which connects Flam and Balestrand as well as going all the way to Bergen) runs only once or twice a day. That's probably the major reason why Rick's guidebook recommends staying in Balestrand only if you have at least two nights in the area.
At least two other posters have written about renting a car and doing some driving north of the Sognefjord. I think both were staying (at least part of the time) in Balestrand. I'm not sure how practical it is to stay south of the fjord (in Voss or elsewhere) and plan to explore north of the fjord. I have the feeling the process of getting across the fjord could chew up a lot of time. However, that is something I've never researched, and my supposition could be totally wrong.
I can't help feeling lodging availability may be drying up as I write this response. I'd go ahead and book something on the north side of the fjord if you think you might want to spend a night or two there. Just cancel whichever lodgings you decide you do not want as soon as you can so others have access to them.
