I've looked at both programs and neither one was what I was comfortable using. I didn't like VDJ at all and I've tried it multiple times because so many people are using it and recommending it all the time. Definitely not my cup of tea. Too busy, too complicated.
I like a simple, clean interface that's easy to navigate and that isn't cluttered up with tons of things that I don't or won't use.
I like a simple set of clicks, sliders or buttons that are intuitive to make the program do the fiunctions that I want to perform.
I know Rockit can be skinned with lots of different skins and so I could probably find one that fits how I like to do things. Mixxx is already simple, intuitive, easy to learn and operate and it's free. If I want to or need to, I can create a playlist in a couple of other programs that I use regularly and just drag them into Mixxx. That isn't a dealbreaker, I was just pointing out that I would prefer the automix to NOT delete songs once they're played.