I didn't think that any AMC drives would go that high? I didn't see your exact drive on their site. But having 120 or 240 as an input voltage suggests that it is one of the AC input drives. For those, it doesn't matter if you use either, the servomotor is going to see the same voltage. I'm pretty sure AMC doesn't make any drives that go to 320.
The advantage of higher voltage for a given motor is the max speed is proportional to voltage. If it is a dc input drive, I'd probably get an isolation transformer and run rectified 120v into the drive.
You need to check your ratings.