Random thoughts and comments as a prior visitor to both:
Weather being equal, personally I would spend more time in Bergen than Oslo, but I like the nautical atmosphere, etc.
This is the porter service I used last year with no issues when I did NIAN:
http://www.porterservice.no/Individuals.html
That time of years, the days will be really long, so take that into consideration.
Given the fast airport express train in Olso from the station to the airport, I am not sure you save much time by staying at the Park Inn at the airport as opposed to the Scandic or others by the station (depending on starting point, walk to the Park Inn is probably around .5km).