This is a very interesting and important question. I think that until we answer your question rigorously, the best bet right now is to start with the expensive

This is a very interesting and important question. I think that until we answer your question rigorously, the best bet right now is to start with the expensive powerful LLM, and then after your system is prototyped and working, piecewise move prompts in your prompt network over to other systems as it proves feasible. If you try to start with the multiple cheap LLMs approach, it is almost certainly going to be more difficult to get the prompting right, and if you don't already know what you are trying to accomplish is feasible, you're adding a lot of work that might add up to nothing (even if it would actually work if you used the more powerful model).