Both benzodiazepines that you mentioned indeed have very different half-lifes. Xanax tends to stay active for about 4 hours but does have a very quick response time (taking dosage until the time you feel its effects). It is for because of the intensity and quick response that it is often prescribed on a prn basis with dosages ranging from .25 mg to 2 mg. Yet because of its intensity, it is a highly abused drug and if you have experience with self-medicating or addiction issues, I recommend that you seek an alternative treatment

Klonopin has a half life about twice as long as Xanax but it takes a lot more response time and the intensity of its effects are very low by comparison. In fact, I dont even feel my dosages at all but it is preventing onset of panic attacks.

Ativan is a benzo that you might want to consider. Its half-life is about 6 hours and the intensity is mild. I have also responded well to ativan but switched to klonopin as I do not want to be mentally altered, even minimally.

I hope this information was helpful. Regardless of the treatment you choose, realize that benzos stay in your blood system between 48 and 72 hours so it is very important that one does not exceed the recommended dosage.

Best wishes,
ptk