Only stayed at the Sheraton so I can't compare the two, but I quite enjoyed my week's stay at the Sheraton last summer. Great location (near main train station - walkable from where Arlanda Express lets you off in town -- and also near shopping and just over the bridge from the Old Town) and wonderful staff as well as nice rooms.

Only caveat is that as of last summer, while they have free WiFi in the lobby and second floor lounge area, they did not have hi-speed at all in thr rooms. Check first if that's an issue.