I may be wrong but it all depends on what your client wants! Has he/she applied for an API or CPI a/c?
The choice comes down to money for the client in the end.
The HSBC Secure ePaymnets Card Processing DVD (impress stock order No. 96301-2 MCP 24494 01/06) will give the client a fairly good explanation if you don't want to advise them on their choice.
Once your client has made a decision, finishes applying to the bank and gets the paperwork so you can set up the module Admin side you should be alright.
Suggest you give HSBC secure epayments tech line a buzz (sorry dont have their number). Alex says that they have been most helpful in the past.
Let me know how you get on if you like.
Thanks!