They'll both work just fine. Xanax hits you faster which is probably its sole benefit over Valium. I find Valium to be more recreational. They'll both bring you down from the cracked out stimulant come-down and they'll both help you sleep.
Edit: As others have mentioned, assuming you have no tolerance to benzos, dosage should be 0.5-1mg alprazolam, and 5-10mg diazepam. As with other substances, it's best to take too little first and then redose later if need be. Try to wait at least an hour before you redose to allow the full effect of the dose you took to sink in before the next dose is taken.