Halcion or ambien (zolpidem)? Which is better for chronic inability to fall asleep insomnia? I was on ambien (zolpidem) from 2009-2014. Doesn't work like it used to. Thx

Neither. These two drugs should only be used short time. Like any other addicting drugs, tolerance to their effects develops. There are better, healthier treatments out there, both pharmaceutical and behavioral. Talk to your doc about alternatives that do not depend on a controlled substance, or get a second opinion.
Tolerance issues. Chronic use of sleeping pills in the end results in tolerance and worsening insomnia. Personally, I don't like either of your choices, and would advise use of Melatonin, Lunesta, (eszopiclone) or trazadone instead. But discuss with your doctor and together choose an appropriate pathway.
Depends . Both work well to induce sleep, they have similar side effects and risks. The difference is in the duration of action/effect. Both gradually impair your ability to achieve sleep without assistance; often gradually require increasing dosages, therefore not best as only agent and technique in the long run. Better to find and fix cause of insomnia. Consider consulting a sleep doc.