one reason many people choose linux for a car pc is because its open, flexable, and reliable, but when you try to write a program that does it all, not matter how well the program is written it looses the flexabiliy of linux.
Maybe a user lives in a country your built in map program doesn't support and they have a seperate program that works fine they want to run, by basing your distro of x and a lightweight wm they can run an external program to handle whatever they want, this is the approach the team working on nghost is taking, and definately what i have an interest in using, and hopefully finding the time to contribute some.
Silverwolf has made some great advancements since the last version, and looks sharp now, but from my understanding it is going to be a standalone program outside of x. for many people that want multimedia proformance in the car this will work great and be easy for them.
but for those of us that want to be able to try new things i'm guessing nghost will draw our attention.
Both teams are doing a great job, and i look foraward to seeing the results.