What is the difference between BABA and 09988? Which should we buy?

Well, not much really.

BABA is US-listed and listed in USD while 9988 is listed in HK in HKD. 8 shares of 9988 is fully fungible with 1 share of BABA.

Personally, I would get the HKEX one if I held Alibaba's shares. The risk of BABA delisting is definitely higher than 9988 delisting (although I would say BABA delisting risk is quite small anyways).

Of course, it's easier to buy BABA if you mainly hold USD-denominated assets since you'll have easier access to USD lying around. That said, buying 9988 diversifies yourself away from falling USD concerns too.

So, which is better is really determined by what's important to you.