I receive live lessons by teachers based in or near New York (12,000 km away). I also participate in live guitar "meetings" online. We use Zoom for these sessions. I also use Skype, Skype for Business, and WebEx for business related meetings, usually involving several participants from various countries, some in developing countries. In terms of video/audio quality, they are all more or less the same in my experience.
Unless you are using corporate systems with a dedicated wide-area network (WAN), any video/audio connections will travel through the normal, world-wide internet. All packets of information travelling through the public internet have the same priority, irrespective of whether its video, audio, email, web-browsing, whatever. The data packets travel through myriads of servers, and data packets compete to use the same transmission lines and servers, often queuing before being sent on. The user experience depends on the local broadband connection and internet provider's levels of service, other data traffic loads, internal modem configuration/type, and house/office wiring. [As an aside, I just had a recent experience at my own home where the internet wiring connections under the house had been made using simple screw (electrical) joints some 15 years ago. The joints had slowly corroded affecting data transmission speeds (down to 0.8 Mbps). Once the joints were replaced with proper telecom gel-crimp connectors, the speed was immediately back to the maximum for ADSL of 8--10 Mbps.]
There will always be a lag between users depending on distance. In a discussion recently involving two people who wanted to practice duos across the Atlantic (USA to UK), someone with a telcom background said the best case, minimum latency would be 200 ms. That is roughly the time of a semiquaver at a moderate pace each direction! Most often, the lag will be more, although still acceptable for one way instruction.
For cases where students are experiencing severe problems with internet speed, you could ask them to do an internet speed test (ping time, upload/download speed). There will be plenty of speed test sites available in most country that operate straight out of a web browser (a Google search will find them easily).
In terms of which system is best for teaching, if the video quality is more or less the same, perhaps consider which is easiest to use for both the student and teacher. Personally, I have found Zoom has the most intuitive, easy to understand interface. The user buttons and features are readily displayed and easy to find. The teacher can toggle audio on/off if there are several participants (helps remove feedback and extraneous noise). It is easy to do screen sharing to display music, to annotate, or display other resources. Ever since an upgrade about 18 months ago, I found the personal Skype interface cumbersome. Yes, the interface looks very clean, but that means user features are hidden away. Doing simple tasks such as video/mic testing, screen sharing, creating URL links for meetings, and adding new participants live have become harder and not intuitive. At least, that is my experience and opinion.