Alipay or WeChat

both is good, though since we also use WeChat to contact each other for social purpose ,so it seems Wechat is more use . 

Caveat: I'm in Beijing, not Shanghai.

IMO, you'll end up using both, but I'd start with Alipay (it's a bit more 'focused' on buying things, and not so much 'social').

IINM, you can just add a visa card and you can just pay for things with a qr code. I added a virtual debit visa card from my (UK) Wise account, where I have already converted money to RMB (I pick when the rate is good) - using the card to buy things in RMB just takes RMB from my account and doesn't involve any exchange rates or fees.

However, to use Alipay on the subway, I think you'll need to scan in your ID (passport), and I've found that problematic. Prior to trying that I was able to buy things from shops/stores without giving ID though. I think I could use it to travel on the bus too. I never tried paying for Didi (ride sharing). All these services are integrated into Alipay, and it works well enough, I find.

Wechat, though, allows for miniprograms and many stores/restaurants have them on there, and you can get things a little more cheaply if you use them (eg McDonald's is a bit cheaper using Wechat than Alipay, I was told).

WRT cash, I think it is a good idea to keep some as 'backup'. IINM, places that accept Wechat/Alipay also have to (ie it's the law) accept cash (not so for online-only stores like Luckin); but, like you have heard, I think people will be annoyed if you present cash, and more so for big denominations

HTH

If you don't have a bank account, carry cash when you arrive in China. Once you have a bank account, link it to WeChat and Alipay.

almost equal convenient for Alipay or wechat if you have enough RMB in your own bank account.   As easy as scan a QR code and confirm to pay !  So, be careful to scan any QR code before paying. 

@luoxiaoxiaoxue

Thank you for your concern.

@mrangry

Thank you for the info. It will be useful for us.

@franklinyang

Thank you for your suggestions.