both great cities! Shanghai is my favorite city in China, it’s definitely the most modern Chinese city. Comparing the two – Shanghai is more diverse and cosmopolitan, it’s also the bigger city of the two (24 million vs 10 million).
Both places are very expensive to live in, but Shanghai is slightly more unaffordable for the locals I think. You will definitely find more Koreans working in Shanghai then Chinese working in Seoul. Shanghai has the most english speaking Chinese in China, and Seoul has the most english speaking Koreans in Korea, I mean they are the biggest cities of their respective countries.
Both have great metro systems, and great nightlife, and skyscrapers but Shanghai has more modern looking ones because it developed later.
You can’t really go wrong with either for travelling or living as an expat it just depends what you want. Shanghai will definitely give you more variety in terms of food, people, etc Seoul just like the rest of Korea you will find people conform to one standard, one look, one style and stick to it. For me, that’s great I like Korean style, but for others I can see why they think Seoul lacks diversity compared to Tokyo or Shanghai.