They are from different classes of medications and have different mechanisms of how they exert their action.
Benzos usually are taken as needed (although some people take them regularly), they work quickly to relieve symptoms and they do cause dependency when taken consistently for a long time.
Lexapro (SSRI) is used for prevention of anxiety/depression, it takes about 3-4 weeks to build up appropriate levels in the blood so that it becomes useful, it causes no dependency and can be stopped much easier compared to benzodiazepine.
Let me know if this answers your question. I didn't know exactly how much detail you wanted so gave you a brief overlook.