I can't speak about other options because I'm not familiar with them. However, the GT1 is what dealers used in the past. It can tell you BMW specific codes/faults. Also, once it finds faults it usually has a Test plan to follow. In this way it will more than likely lead you, through testing step by step, to identify the problem(s).

Failing that, go to a shop/mech that is familiar with BMWs/X5s specifically and depending on what diagnostic equipment they can do similar diagnostics.

Diagnosing properly is the key to prevent wasting time and money.