Its good that SAP is your interest. But you didn’t mention ur domain experience / skillset.
The future belongs to SAP. You can select any domain but your back ground should match the domain.
If you are into finance you have to select finance and controlling, if you are from a healthcare back ground, then go for SAP healthcare.
none of the domains are comparitive. It entirely depends on your qualification and experience.
If you are specialist in a particular area and get SAP certification in the same area, will definitely help you further in your future.
Please mention in detail, your domain experience and qualifications in detail.