My vote is voice for all with existing controls for backup.
So long as you're getting around the need to touch the touch screen, how about making the NAV destination control available while moving?
And how about adding XM surfing commands like, "radio, channel four six" and NAV commands like "map zoom/unzoom/pan left/right"?
Onstar commands work pretty well for me so I imagine your control systems will do fine. Madam Onstar did get a little pissed at me once when after three consecutive "Slower please" responses from her I told her to SHUT UP! She said, "Pardon me?"
