what card we will use for HK transportation?

Hi, as you also wanna use the same card in Shenzhen, I suggest you but Hu Tong Xing, which is a combined transportation card. It can be used both in Shenzhen and HK.
As I know, it costs CNY80 in HK and you can buy at the Customer Service Center of most MTR stations.
A reminder is that when you use it at HK, you need deposit with HKD to use for HK transport. Similarly, you need deposit with RMB to use for Shenzhen MTR.