Oslo or Hamburg?

I love both cities, so this would be a very tough call for me. Hamburg has all the vibrancy of Berlin or Munich, but I think it is much more attractive than either. It has some pretty good museums, but if you just want to stroll around, the area around the Binnenalster is one of the most elegant pedestrian-only shopping districts I've seen in Europe. The city's Rathaus alone is larger and more elaborate than many countries' national parliaments. For model railroad fans, Miniatur Wunderland is an absolute nirvana and mecca in one building. I would rate Oslo as one of the cleanest capital cities of Europe, and also one with the most scenic geographic setting. I really enjoyed the Norsk folk museum and walking around Frogner Park. Downside- very expensive! Your money just doesn't go as far here as it would in Germany.

My take is different from Tom's. I consider both Hamburg and Oslo "second tier"; I don't regret going to see them, but don't feel a strong need to go back. So, in your situation, I'd look at some guidebooks or websites to see the attractions in each, and see which one has more things you want to see. I, personally, found more to do of interest in Oslo, but that's just me. Tom is also correct that the prices in Oslo are MUCH higher than in Hamburg. You should look at Rick's suggestions for Oslo and Bergen, as he has some ways to save (a bit of) money there.

Since you're already going to be in Hamburg you might as well stick around, there is plenty to see and do. I visited the beginning of May and loved it. I spent two days there and still didn't even see everything, just the new Hafencity district and the Harbor area....never made it to the city center. Minitur Wonderland model exhibit is a must-see attraction, one of the most unique/amazing things I have seen anywhere in the world. For me ranks right up there with the Grand Canyon, Honk Kong skyline, Eiffel Tower, St Paul's Cathedral. Haven't been able to stop thinking about it, and am planning on visiting again in the near future. It gets very crowed mid-day but if you visit a couple hours before closing you can have the place almost to yourself. Also nearby is the new Maritime Museum. It's top-notch (start at the top floor and work your way down). I had a great stay at the 25hours Hotel HafenCity design hotel. It's got a very interesting maritime theme. Check out Rick's excellent guidebook for the region, he got a chapter for Hamburg, it's got all the must-see attractions covered: http://tinyurl.com/mdy93cc

Ditto the recommendation for avoiding Minatur Wunderland mid-day. I arrived first thing in the morning and I enjoyed relatively unobstructed views for about the first half hour. By mid-morning, though, the place was packed... and this was in the middle of the winter, not even remotely the height of the tourist season. The layouts have so much detail that you really need to get up close to fully appreciate them. Very difficult once all the families with children show up... I'm not going to shove a kid out of the way so that I can see more clearly... I've also visited the above-mentioned Maritime musueum. It's not necessarily as engaging as Minatur Wunderland, but worth a look if you have any interest in boats and such. And it's also huge. At a quick pace, it took me more than two hours to go through.

Hamburg has plenty to see. Hamburg's Rathaus is IMO much nicer than Oslo's city hall. Hamburg also has a bustling port that you can tour by canal boat. You can visit churches that Martin Luther preached in. As others have mentioned, Hamburg also has the miniature world. in August 2014, some bars on the bay also create a beach bar type atmosphere. Oslo, OTOH, has their city hall (modern), Nobel center, a nice walk through downtown, and Akershus castle (I didn't tour, according to Rick it's pretty average). Frogner park is great and my top sight in Oslo. Oslo is much smaller and may be the better one-day visit. You could spend more time in Hamburg and not be bored.