Hi BP,
If you have already working on SAP HR then there is not specific to be trained / worked on. Each country has unique payroll functionalities and structure of recruitment process.
However, if you calculate the implementations in SAP, then US is best option to learn US Payroll system.
This is purely depends on which project you work on. Normally, when you go to project you will learn depends on country implementation. If you are in rollout project, then you can learn each country payroll system every six months.
Again, if you are looking for onsite then US and UK payroll will take you to onsite projects because the projects are interested to take already experience guy rather than non-experience.
Regards
GGOPII