The prettiest ferry segment along the Oslo-Bergen route is the Naeroyfjord, which you see on a ferry from Flam to Gudvangen--one of the key components of the Nutshell route. The long express-boat trip from Balestrand to Bergen doesn't go into the Naeroyfjord and would not be my preferred way of traveling on to Bergen. You'd need to dig into transportation schedules to figure out how to incorporate a stay in Balestrand (by all accounts a very attractive little place) and not miss any of the scenic segments on the Nutshell trip--which include the Flamsbana from Myrdal down to Flam, the aforementioned Naeroyfjord ferry and the bus trip from Gudvangen up to Voss (where you can pick up a train to Bergen).
The first thing to do is check on lodging availability in Balestrand. The fjord area is very popular; demand for rooms exceeds supply.
Bergen is a lovely town, worthy of a couple of days.
The Geirangerfjord is a second fjord area that's very popular, but public transportation is very tricky; both long-distance ferries and buses are slow and infrequent, making it a challenge to piece together an itinerary. You might consider renting a car from Balestrands (doable if you plan way ahead, I believe) and using it to see some of the area north of the Sognefjord. You'd probably have to return the car to or near the same place you picked it up, though. Having a car for part of the trip would probably make it easier to find lodging.
