I'm Citigold in the US and the UK and also an HSBC Premier customer.

I prefer HSBC, some of the reasons being :

Once you have qualified for Premier in one country you are able to use their Premier facilities in any county whereas with Citi you have to meet their minimum balance requirements in the particular country to qualify for gold in that country ;
HSBC have dedicated Premier lounges and their staff seem more responsive ; HSBC seems to be more of a worldwide bank and seems to be growing whereas Citi seems to be contracting

But Citi does seem to offer better interest rates.

Just my 0.02.

Andy.