Adam,
i can not tell you much about the pixelation because I do not work with AVI files, but you started with a codec that work in the YCbCr color space and you convert it in other that work in RGB. when possible try to keep in the same color space to avoid problems. Exporting in Millions+ you dont gonna get more quality than in Millions. The only difference is that Millions+ support Alpha Channel,, so you should use it only when your film have such channel.
Cheers,
rafael