Hi, I have opened Citibank basic savings account locally in Malaysia and you do not need to link to overseas Citibank account unless it is different account type like current account.
I am able to use my Citibank ATM card to withdraw money from any Citibank ATM overseas and they do not charge for overseas withdrawal fee.
The currency exchange fluctuates according to the Visa rate.
But I will suggest you to check with the local Citibank officer as it there may be chargesor services may varies depending on which country you are going to.
I do not use HSBC, do not like their service.. so I cannot comment or compare. However, Citibank is good enough for me so far