I'd love to help you decide between either the AOpen MiniPC or the Mac Mini, but I don't know that I can. They both have beneficial factors and the true deciding factor is what your looking for in a CarPC.
Personally, I would recommend building a custom system, but if thats not an option I would purchase the Mac Mini. Macs are beautiful pieces of hardware, and OS X is an amazing operating system with some great features that Windows simply cannot match.
As for your question about front end applications, Roadrunner, probably the most popular front end software is only available on Windows. You can still use it on your Mac, but you will have to use either bootcamp or virtual machine software like VMWare or Parallels. StreetDeck, another popular front end application is also only available on Windows.
However, there are a few front end applications that have been developed for the Mac, such as QCar, CarFrontEnd, Neon Boombox, and AMP. If you do go with the Mac Mini, you can always try those out before running Windows.
Best of luck to you!