The radisson is the most convenient as it can be reached by a 2min walk when exiting OSL at arrival level and heading towards the right. The hotel itself is nothing special, and a rate >150 Euros per night (which they often charge) is expensive for what you get. All hotels in Norway have breakfast included in the rate. Gold members get a business room. Dont have any experience with the bar or restaurant there. If you dont mind a bus ride, Thon has several hotels 5-10mins away from the terminal by bus. Rooms are comparable to Radisson, but breakfast is inferior.

The airport express is very reliable and takes <20mins to Oslo S (the central station). On the way back it arrives directly at the airport and with a full train and people moving up the stairs, you will be at your baggage drop within 15mins from arrival at the latest.

If you have never been to Oslo, book yourself into the Radisson Plaza or Clarion Christiania, right by the train station and explore the city. Zou might end up at the same or cheaper rates and have time to explore the city. (If its worth to you getting up an hour earlier the next morning)