A channel is what installs to your Wii menu, therefore a forwarder is a channel. A full channel with the .dol installed in it is suppose to be independent of a storage device (SD/USB), but now a days just about all apps, including emulators, require additional files on the storage device. That kind of makes a full channel redundant, since the storage device is required anyways.
When using a forwarder, the application is easily updated, just replace the boot.dol that is on the SD card. When using a full channel, to update you need to remove the channel and find a new updated version of the channel, and with some of the emulators they want to do an automatic update thru Wifi, which can create problems if you are using the same outdated channel every time.