Demul is definitely superior to Null DC it also has the ability to run a nice section of Arcade games (unplayable through MAME) also so it has that added bonus. I'm unsure as to the latest version of Null DC I haven't used it in a long while. I don't think either support .iso but don't hold me to that as I'm not 100% on that .cdi and .gdi are supported by both I believe. This link will probably tell you more than I can