I seem to remember the key benefit ZMODEM had over Kermit was the ability to restart an interrupted file transfer. This was especially useful in the days of yore when your younger sibling or mom picked up the extension causing your modem to drop its connection.
As I recall it Zmodem was more than nominally faster than Xmodem but in those days to squeeze even 5 to 10% speed improvement in speed on a 2400-14400 bps modem was huge.
