From the TP-Link product spec:
It has a Gigabit Ethernet Port - that's where you connect your your PC/laptop/whatever device) and it can transmit 500Mbps over powerline.
As for 10/100 and the rest - as you said, it's related to Ethernet standards, but in simple terms just tell you 'speed' a device can operate:
10/100 mean either 10Mbps or 100Mbps
10/100/1000 - 10Mbps or 100Mbps or 1000Mbps etc.
Now, when you connect two devices, they will operate at highest common speed (one is 100, one is 1000 -> operate at 100). If they do not support such (say one is set to only allow 10Mbps and other only 1000Mbps) they will fail to communicate.
Going back to your question - it's similar logic here: 1G/500M plug will limit you to max 500M, but 100M/500M (non Gigabit one) will be max 100M as that's lowest speed along the transmission path. If the price difference is small it makes sense to go for Gigabit one - unless you know that none of your devices will ever support Gigabit.
Edit: I have not used those devices, but just googled a bit and it looks those high speeds (over power) are a bit (cough) overinflated. So if you have a chance it would be best to actually try both and see what actual speed you get!
Blackbeagle's answer was quite close, but not accurate after all. I've dug into something that I thought was probably unrelated, namely that the wires had an odd sequence, and that turned out to be the culprit! See photos below.
The essence is that the wall sockets (I took them apart) seem to be wired poorly but in the proper sequence, and the patch panel is wired according to the same standard... but somewhere, the labels must be mixed up because if I switch the wires around in the patch panel, it suddenly works!
Embarrassing admittance: In documenting this answer, I take a close-up photo of the patch panel's internal connectors and their labels. To my embarrassment, I notice that it's not labelled
1,2,3,4,5,6,7,8
but rather
1,2,4,5,3,6,7,8
! Can you spot the difference?
I've rewired one port accordingly and now it works with gigabit speed! (This is why I didn't want to accept Blackeagle's answer.)
The wall socket is poorly patched but the sequence is okay:
Experimentally swapped the wires on one port:
That worked! Now I need to do the same on all the ports. That's when I discover...:
What?! The sequence is not as I thought it was. It's my own fault and I've been overlooking it all this time. I am embarrassed :-)
Best Answer