Benedryl is diphenhydramine and Unisom sleep aid is doxylamine succinate. They both are effective, OTC, and safe if used as directed. If you go for the unisom make sure it's the sleep aid with doxylamine in it, as they are now selling a unisom version with dyphenhydramine.
I do not know this but I wonder if you swap out one for the other every other night or rather as needed if it will keep tolerance to each at bay for a while. It's just a supposition, but might be worth some thought.