The only difference is the firmware you put onto the Dongle PIC chip.
The hardware is the same build either way.
If you put on the pixelnet firmware for the DMX Dongle, you then can connect that Dongle to a Smart String Hub (SS HUB).
From a SS HUB, you can optionally also use the DMX512 port on the hub (and can select which 512 are shared between pixelnet and DMX).
If you put the pixelnet firmware on a DMX Dongle, you pretty much need to have a SS HUB and likely also SSC (smart string controllers) and
DMX items ... can mix.
If you put DMX Dongle firmware on the DMX Dongle, then its a straight 512 channel DMX dongle and you can connect only to DMX items ... like Lynx Express controllers and any other DMX items you might want to run. You will not be able to use it for smart strings. But of course, you can use it for a season as a DMX dongle and then the next season as a pixelnet dongle by simply re-flashing the PIC.
Your right, the RGB smart string items don't use triacs and are 12v also ... using the SS HUB for centralized power supply (typically). If you happen to run a Lynx Express (LE) ... then those are conventional AC controllers which use triacs ... and are just DMX controlled. You can control then via. DMX or via. Pixelnet using the DMXOUT port on the SS HUB.
RJ and his designs give you lots of options.
Hope that helps.