Both the Clarion and Radisson Blu Sky City are both inside Sky City. The Clarion is about 75 paces down the same hallway from the Radisson. The Comfort is probably 400 paces (and two outdoor street crossings) from the Clarion.

The Comfort is very basic (but not super basic like Good Morning Arlanda--which I'd also recommend if you just need a place to shower and sleep). The drinks at Comfort were good, and the breakfast was sufficient. The Clarion has a fantastic breakfast spread, and Kitchen & Table on the top floor also has decent food and drinks.

Since you are traveling with just a backpack, I think the Comfort will be a fine choice. If I had to lug multiple checked luggage around, then I'd spring for the extra SEK 400 for the Clarion.

Edited to add: here is a thread in the SK forum that has additional information... ARN Airport Hotel recommendation