I have a UNIPORT, just like the one in the picture. I couldn't get the charge pump (PC present signal) to work with LinuxCNC, I think I had the same trouble with Mach 3 too. Apart from that there is not much more I can say, totally faultless, transparent in operation, in fact I just had to check to make sure it was still there

The UNIPORT is powered from the PC by a USB cable, or some other 5v supply.

It looks like the OPPB requires a 15v or above power source - you can see the big heatsink which probably cools the onboard 12v regulator. It has outputs providing 12v and 5v if you need them.

The UNIPORT has five optical isolated inputs, you can see the row of little 6 pin chips. Optical isolation helps protect the board and you PC from any nasty external voltages.

It looks like the OPPB has optical isolation on all connections back to the PC (?) and not just the five external inputs, a full 17 little 4 pin chips in fact!

So, OPPB has more flexibility with respect to power supply and provides the attached PC more protection. I think.