The luminescence method is the most sensitive one by far for detecting ATP. This means you can use the smallest amount of bacteria and/or the least amount of ATP release. If you want to use another method, you will need much more bacteria. UV absorbance at 260 nm is not specific to ATP.
I agree with Adam. Whether ATP or ADP and AMP leaks out depends on the action mechanism of the antibiotic. If you just want to see the leakage I suggest measuring ATP+ADP+AMP continuously by adding the required reagents to the cell suspension. I can provide instructions if you send an email to [email protected].