I too am debating the choice between the Audiovox and the Moto. I've never used either one, so I can't give you my own opinion. But, in case you haven't seen it, here's a discussion on the same topic:
I am trying to choose between the MPx220 and SMT5600 but am not sure which one's better. I have heard the concerns about the MPx220's volume problems but like the clamshell design. Most of what I read about the 5600 seems positive. Does someone have any input regarding these two phones? Should I just wait for new phones to come out with Windows Mobile OS?