Hello,
It depends what you want to do with. I think that coldFire is suitable for industrial applications, IMX series for multimedia applications and Power architecture for communication and high end automotive/industrial applications.
First, you have to know, how much money would you spend for processor, 2nd do you really need to run linux ( if no, there's alternative solutions like RTEMS, FreeRTOS, uClinux), and do you need real time applications (probably no, because you want to run linux
), but, if yes Coldfire is more suitable for that. ARM architecture is suitable when you need to treat high data volume with complex algorithm (Power architecture's too).
Freescale web site, can also be a good help to do your choice.
Emmanuel