I agree that ChaCha seems like a better function (designed for faster diffusion, better block layout, etc), and it is comforting that fewer ChaCha rounds can be attacked with the same attacks that apply to Salsa. But ChaCha does still represent a substantial change in the core function from Salsa, and I do wonder if it's possible those changes introduce a problem not found in Salsa. To be honest, I really suspect the answer is no, and that ChaCha is a good choice...just surprising given the relative interest in Salsa compared to ChaCha. But maybe your choice will also spur more people to consider ChaCha for both usage and further cryptanalysis.