MTU will set the maximum packet size before it will fragment the packet. IP MTU will set the maximum packet size for IP packets before it will fragment the IP packets.

From cisco command lookup tool....

Changing the MTU value with the mtu interface configuration command can affect values for the protocol-specific versions of the command (the ip mtu command, for example). If the value specified with the ip mtu interface configuration command is the same as the value specified with the mtu interface configuration command, and you change the value for the mtu interface configuration command, the ip mtu value automatically matches the new mtu interface configuration command value. However, changing the values for the ip mtu configuration commands has no effect on the value for the mtu interface configuration command.

Hope this helps,

Mike