I've checked the Carnival & the front seat is very roomy, very nice. But behind the front seat it feels oddly smaller than the Odyssey, might be because of the headroom.

The Carnival doesn't have a rear camera, so if you're client is driving the car, Odyssey might be better. If he/she is being chauffeured, I hear the Alphard is really cushy & quiet.

Caveat with the Odyssey are: ground clearance might be iffy; reviews mention of stiff, harsh ride on poor road surface; it's really only a 6 seater-unless you put very slim persons on the third row; the head unit & A/C control are will definitely get scratched; and that special A/C condenser is bound to be expensive to replace.