Having the default transposition model be isotropic seems like a very poor choice. Isotropic models are known to have very large biases and should only be used in very specific use cases (generally not be new users).

I did a brief survey of commercial software and found that the vast majority only offer the Hay-Davirs and Perez model, with the 1990 Perez model seeming to be favored as the default model.

It would seem to make sense to have the same default model used in pvlib.modelchain.ModelChain.