Hi Mahesh,
Both SAP Security and Basis are big oceans. Learning never ends in every aspect. However, if SAP Security is what interests you, then you go with it.. There is indeed lot of scope in this area. Basic knowledge of Basis Administration would be an added advantage for Security consultants., and vice-versa..
SAP Basis involves everything from planning to implementation of SAP architecture (both SAP and its infrastructure platform), maintenance of SAP systems with varied SAP components. Whereas Security just handles the security and auditing part of it which involves, security role designing to component based security implementation and further advanced areas. SAP GRC and IDM plays important role too..
Basis guys usually take care of installations, builds, upgrades, maintenance and other admin activities..
Go with where your interest lies.. Demand is everywhere! :-)
Regards,
Shahid