badi is objective oriented but user exit is not

In USEREXIT u code in the standard SAP program, hence any 
updation in the version of the standard program will lead 
to the loss of ur coding. But same is not the case of BADI. 
Here the code remains outside the standard program. 
You need to search for the suitable BADI as ur requirement 
and then do the coding and plug in the program. 

USEREXIT - It is also a program enhancement technique. here 
also u need to find suitable userexit and code in ur 
program.

BADI is OOPs concept...but User Exit not
BADI can be used more than once... but User Exit can be 
used only one time
BADI code remains outside the standard program. 
User Exit - code will be included in standard program