LlamaIndex allows you to easily get started with querying over your documents, as well as using agents/chatbots. You easily customize every component in the query pipeline as well
Langchain is more of a "kitchen sink" library, which in my opinion makes customization a little hard
Definitely recommend reading the high level guide for llama index here
https://gpt-index.readthedocs.io/en/latest/getting_started/concepts.html