It doesnt matter if you take the 121 or 135 version. Your ATP is an ATP whether it is to be used for 121 or 135 ops. I had the same hesitation when I took mine (flying 135), but the FSDO and Simuflite both said it didnt matter which one I had taken, as long as it was done and at least a 70.
Your FSDO, if you fly 135, could better answer your question. If you dont fly 135, and want to go that route anticipating a charter job, I would just take the 121 test.