As an affiliate, I would choose Microgaming casinos before Playtech casinos at any time. It is my undocumented experience that they convert much better.
I would agree that they MG can sometimes lack a bit in graphichs compared to e.g. Playtech, and personally I like to play Playtech slots.
NetEnt is probably my prefered slots developer, but casinos using this software is not available for US players either (actually, I didn’t think Playtech was available in the US).