Which weekend in mid-May? I voted for Stockholm, because I think there's more to do in Stockholm and the prices are cheaper if you're wanting to shop.
However, 17 May is the Norwegian national holiday. It can be a little inconvenient as a tourist because the city is crowded and shops are closed (Restaurants and some museums are open), but I always encourage people to visit then if they can. Pretty much the entire city turns out to celebrate and wave flags, and a lot of people wear bunad (the traditional costume). Since Oslo is home to people from all over the country, there's quite a variety, since each region has its own bunad. If you're talking about that weekend, I would definitely consider it, because it really is a fun sight.
I haven't been to the others, but I really enjoyed Stockholm--and I was there in January, so it was rather chilly. I enjoyed wandering around and popping into coffee houses to warm up. One of my favorites was Cafe Kaffekoppen www.cafekaffekoppen.se/, which is in the heart of the old district.
My sister was in Oslo and it was one of her least favorite European destinations. I'm sure it didn't help that she was on a budget, but she felt like there wasn't as much to do as other places she visited.
GilliC and I are bad former and present Norway dwellers, but for real stay away from Oslo on May 17.
Hahaha yep, I never thought I'd be that kind of fan, but I figure tagging on a weekend trip doesn't make it seem as crazy.
And both London shows sold out in minutes, so I never even stood a chance. I was able to get pretty decent seats in Stockholm for less than the really cheap seats in London!
GilliC and I are bad former and present Norway dwellers, but for real stay away from Oslo on May 17.
I like May 17! I think it's cute. It's definitely not a great day for doing traditional sight-seeing, but I think it's fun to see everyone out in bunads and actually waving flags!