Is it better to join after graduation or after MBA?

You can join after graduation. If you have an MBA, you may get some advantage in salaries in some cases. Salary after MBA also depends on the internal grading of your college, that may vary from bank to bank. In general, an MBA degree will help your early career, only if it is from a reputed institution. The slight advantage in salaries that most MBAs get will be more than offset by the salary growth and real experience gained during those 2 extra years.