Go for SAP ABAP. As per ur background, I think ABAP is suitable for u. Before taking decision, u have to discuss with ur close persons, and if possible consult with a renowned consultancy which deals with SAP. For better info :
About sap career
SAP ABAP or SAP Basis?
For Mainframe: http://publib.boulder.ibm.com/infocenter/zos/basics/topic/com.ibm.zos.zmainframe/zmainframe_book.pdf