We used Hertz. If you are on a cruise, the Hertz 'shop' is right under the bridge. On the water. Just walk along the water walkway.
In my opinion, I would get the car. It was easy and we saw a lot more than being on the ferry/bus.
And, yes, we took the car on the ferry.
We got the ferry from Brensholmen at midday. You just line up and drive on. Cost NOK 280.
We had lunch at Hamn i Senja. It was wonderful!!!!!!
So,we left Tromso around 10am. Drove to Sommaroy, looked/walked around, then drove to Brensholmen (about 10 minutes from Sommaroy). Got the ferry, drove to the little seaside village where we had lunch.
Drove down to finnsnes (stopping along the way) and then took E6 back to Tromso.
Got home around 9pm.
SO weird having no dark!!!

forgot to add.....pack a picnic lunch/snack/drinks. Restaurants are few and far between!!!! The place we went to was the ONLY one we found in our 10 hour trip.