Hey. Firstly, congratulations. You're in a good position.

I work in Shanghai and I've been to Tokyo many times. Straight away I will tell you this: You should choose Tokyo. Below are my reasons.

Shanghai is probably China's best city overall. There are prettier cities (e.g. Xiamen) but these pretty cities tend to have no jobs and very little nightlife. Also, these smaller cities tend to attract 'weirder' expats. Sorry to say that.

China drives me crazy. If you look at most of the other posts here on /r/china you'll see that's a common theme amongst expats in China. There's a number of reasons for this, and it's not things like pollution. In a nutshell, Chinese people are really shitty to each other (except to close family), there is almost no quality control anywhere, constant scamming and genuine increased cancer risk etc. from fake food/alcohol/water/cooking oils/etc.

I'll give a little example. You want to get some milk in the local shop. When you leave your apartment, you see cigarette butts on the ground. The lift comes up, and it's wobbly as fuck. Someone's been smoking in the lift. As you exit the building, you see the security is asleep. Walking down the path, you have to walk on the road because everyone has parked their cars on the path. At the green man, you have to stop because a car drives through the lights. You make your way to the shop and get some possibly fake milk. The girl working there ignores your ni hao and xie xie, and overcharges you.

Every day there is shit like this. It's constant. It grinds you down.

Now, Tokyo. It's the opposite of this. More expensive, and a little lonelier (Japanese people are more standoffish), but almost everything is better.