I currently have two online games going, both RPGs (Pathfinder 2 and D&D 5e).
For one we use Slack, because a member of the group has a business account. I understand it has a small fee for the originator of the audio/video call. The GM uses a separate camera attached to his computer to provide an overhead shot of battle mat when combat occurs. We just roll physical dice and report the results, and track our own physical character sheets, As of yet we haven't mapped anything, though the GM has e-mailed scans of rough maps of the adventuring region.
For the other group, we use Discord for the audio/video call and Roll20.net to handle maps, character sheets, rules details, dice rolling, and combat maps. We haven't done much in the way of physical mapping ourselves. (This group is made up of teens and run by a teen DM; as I recall from observing them play over the last few years, they never seemed to do much mapping, so it may not come up.) Discord and Rollnet.20 are free.
My reviews:
Both work in a similar manner, at least as far as user experience. It's not always obvious what you need to do to get the call started and turn on the video and audio capabilities, but it's not hard to figure out, either.
Discord offers an iPad/iPhone app (and presumably Android and Chrome too) which is free, or it can operate on a browser. I use my iPad for the Discord, and for me it works great.
I use my desktop Mac computer to access Roll20.net. I will note that some elements of Roll20 don't work well, but these are largely tertiary features that I don't need. The problem may be on my end, as I use Safari. Otherwise you have to click around a lot to call up the different features, and the browser window can get cluttered if you try to have several things open at once.
I run Slack on my desktop via a free desktop app. It works great, but as I said, some of the features and icons aren't intuitive as to what they do.
Both Slack and Discord allow you to turn on your device's camera for video calls. You can leave the camera off and just to audio. You can mute your microphone independently, too. As a default setting, Slack shows each participant as small square icons side by side (the icon is either live video or a selected avatar image, depending on the participant's camera setting). The system automatically zooms in on whoever is speaking the loudest (or makes the loudest sound, even if accidental). This can probably be set to different approaches, but I haven't tried that yet.
Discord splits the screen into rectangles based on the number of participants. Each rectangle shows either the participant's video feed (if turned on) or a unique color and round avatar icon in the center. If the latter, the icon will be surrounded by a thin green circle when the icon's participant is speaking.
Other than that, they're functionally identical.
Discord has some security issues, but they're largely cases where you have to agree to a request sent to you by another Discord member, who might actually be a bot or spammer out to spoof your account. As your mother taught you, if you don't know who it is, don't answer the door.
As far as I know, Slack is secure; it helps that it's not a popular gaming platform, so maybe has a little less gullibility and more maturity in the clientele, and no paths for uninvited guests to show up.
Since Discord is free, it's probably the better option, but otherwise both work.
