To the best of my recollection from articles i have read the choice in new designs is as follows:
Simple tasks (e.g. a uC in the rear that recieve serial info and control taillights): a simple 4 or 8 bitter e.g. a PIC.
Medium tasks (e.g. charge control): a more powerful 8 or 16 bitter.
The main machine: 16 or 32 bits.

The tendency seems to be towards a lot of small controllers and ONE powerhouse, rather than one or a few central controllers.

Erik