I have both and will keep both. Even after 1 day with the mini, I can see that they are different devices.
The Air will get some light productivity (nice design tools from Adobe and others), some video, lots of surfing, as well as an app development test device.
The rMini will live in my purse for true portability. It's primarily for e-reading and music... Apple finally released an iOS device that can hold my entire music collection! I don't think we'll ever see a 128GB iPod touch, so this finally retires my iPod classic.
As far as usage patterns in general, the iPad was quite a revolutionary device compared to the iPhone, both in display size and pixels. It enabled a lot of new apps that could never have worked on a phone-sized device. With the rMini, the pixel count is the same as the Air, but it does feel like it would be cramped for some apps.