i am a fan of infinity although there is nothing wrong with mixing and matching. There is no difference performancewise between dual or single however it reallt depends on your amp. If your amp does 600 at 2ohms and 400 4ohms then you want the dual 4ohm so that you get the most power. If you want to maximize the usage of your amp and sub which may not always be the best idea. Do some more research and find a couple of amps you are interested in and then decide. Some amps arent 1ohm stable and if you are going to run two subs that might be a problem for dual 4ohm.