Xanax is essentially the quickest acting sedative in popular use. Quick onset means it dosen't last long though (also makes it very addictive). It's great for panic attacks. Klonopin probably lasts roughly twice as long (with slower onset). If you use either medicine often enough, tolerance can develop. However many people take these every day for years with no problems. We prefer to treat anxiety over the long-run with an SSRI antidepressant though (Paxil, Lexapro, etc.).