user exits vs badi

hi all, one of my friends who is working in real time said to me , whenever a need for enhancement arises, its always better to use badi over user exit. he said , if v add our code in BADI , and in future if any problem arises with BADI , SAP wil take care, but same cant be done if v use userexits.

am quite confused.plz give the basic differences about badi and user eixt, and which of the two is preferred in REAL TIME and why?

thanx all.