i'll answer that.
Asus firmware:made by Asus
Asus made some improvements and new functionality along the way
They also restricted a couple of things because the FCC told them so.
one of them is limited range after firmware 374
so the higher the firmware version , the more functionality but after a certian version(374) range went down the drain.
Merlin Firmware:
bettter and improved version of the standard Asus firmware.
Merlin adds new features to the current Asus firmware but does not change standard functionality(so if asus range is bad, merlin range will be as bad)
However if Asus made a mistake in the firmware(a bug) you can be sure that Merlin fixes it(how is that for support
)
lucky for us Asus knows they are not brilliant and uses those patches to fix their own firmware.
You might call it Asus proprietary firmware with a open source community which has more knowledge about this firmware then Asus does
John's fork firmware v374
John took the last populair firmware version from Asus which was not limited by range.
For a lot of people this firmware is the best regarding range and performance.
John does not alter this firmware a lot but he keeps on adding the latest security patches from the original Asus firmware.
in short:you get the firmware with the best range, compatibility AND with the latest securityholes still plugged and applied.
the total story might be a little different but this is it in a nutshell.