you have pretty much summed it up as far as offical front ends go. qcar is the one with the most potential, but amp works really well for the most part. there are other options, for one you could use the program parrallels, running in coherance mode will let you use any front end from windows basically as an app on mac. this does require a copy of windows to though. you can also use native mac apps such as itunes and front row. both work pretty well with the use of a rotary prehperial such as a griffen powermate or nulooq. you can also write applescripts to help make these apps more car friendly. if worse comes to worse you could always do a dual boot with a new intel mini.
hope this helps somewhat in your decision.