Might be a bit partial, but I'd say Stockholm.
I've been in Olso twice the last 6 months and Stockholm a handful times in my life.
Oslo is a beautiful city that can be likened to a "village city"; it's small enough to walk around the city center in 15 minutes, and you can be surrounded by nature 30 minutes from the city center (by tram). Great public transport and friendly people. Norway is starting to get famous for being a fat nation, but you'll have plenty of opportunity to chat to beautiful women with a happy demeanor.
It also might be the most expensive city in Europe; a simple lunch will run you 30$, beer is minimum 12-15$ in normal places. Norwegians have made it a habit to buy alcohol in Swedish border shops.
Stockholm is
filled with history. If you like old baroque/renaissance buildings and having history around every corner, the Swedish capital will be a treat to you. It's not for nothing it's called Venice of the North.
Stockholm the only proper "big city" north of Denmark's Copenhagen. It's a bit expensive but not even close to Norwegian levels. Beer starts at 7$. Lunch 15$. Like in other threads on the forum, you will find Scandinavia's finest women in the center of Stockholm. They know how to dress and paint themselves.
Thanks to our equal society, with the right game your chance of ONS will be OKish. Downgrade cocky Anglo game a bit when you're here; just be a cool guy with stories and a good vibe. As soon as you isolate a girl you can escalate quite well, just bear in mind that #MeToo is still in full swing in more liberal parts of the city. People in Stockholm don't want to admit it, but most of them are still a little starstruck with Americans and Brits, so you will probably have a little exotic edge.
Like anywhere else, when you visit these cities: Know how to dress. People dress well in Stockholm particularly, but you will stand out if you know how to choose clothes with the right fit. An understated, minimalistic look is what have and always will work in the northern latitudes.
Regarding nightlife, I haven't visited the only true night spot in Stockholm called Stureplan (a small plaza with a few high-end clubs), but I remember really liking a bar called Grand Escalier close to it; was there with a few forum members during the 2016 meet-up craze. There's also a hipster/younger area called Södermalm with a lot of choices if you like students and that type of crowd.
The only general drawback is, that during summer months many natives in Stockholm flee due to the tourist invasion... That is, if your main goal is to score a Swedish flag.
A general recommend is that, if you plan to hit the bars and clubs, buy alcohol in the Swedish spirits store "Systembolaget" and build up a nice buzz before going out. Vastly cheaper than in the nightlife. Apropos that, Oslo is called the Champagne City because buying champagne bottles in the club with your friends is much cheaper than buying individual drinks.
I talk alot about alcohol but you should be aware of the prices before coming to Scandinavia, tourists always get a shock.
PS. If you have the chance to go to Gothenburg the day before it'd be awesome to buy you a beer. You're a great poster around here, and it always easier to give intel 1 on 1. I'm off to Germany on July 20.
Much more to say, these things are off the top of my head. If you have further questions don't hesitate to PM me.