I think a lot of people see a larger gap between open-source models and GPT-4 than is present because they test out the 7B models which fit on their machine. Models like T5-FLAN-XXL are very far below the quality one can expect from the best in open-source, and barely usable for CoT or tool use.
Especially for LangChain, I recommend using ≥33B models like Guanaco or WizardLM. Guanaco-65B honestly feels on-par with ChatGPT-3.5. (To be clear, there is a large gap with GPT-4 though.) It is a costly test, although GPTQ (for instance on exllama) help make it affordable.
I haven’t tried Orca since they haven’t released the weights yet, but it doesn’t seem like they have a 33B version.