SAP BI is evergreen, Compare all other modules in SAP. Coming to your case, you can learn SAP BI. But initially you feel difficult. dont worry about carrier i have observed this SAP BI, people got interview calls in ression also. I think you had java exp. of 3.5 yrs. so SAP EP(Enterprise Portal) also suitable for you which is java programming all most. If you want quit from java then go for SAP BI or SAP ABAP(programming). sap abap will be advantage for SAP BI.
me also very much confused in my carrier, so i am suggesting you that what ever you are mentally happy and satisfied do that thing other wise you will be haveing very sad face and memories in your future.
all the best. JUST DO IT. THAT’S IT.