Good question. Given the current state of affairs wrt Covid19, lots of people looking. Here's what I understand:
One of the big differentiators is where the VTT is hosted: Server or private machine. I.e. Roll20 (and IIRC Astral) use dedicated servers everyone logs into and the game is run there. The systems do not have individualized gaming rules, making it easy to run whatever you want. The community provided characters sheets on each system can be complex, allowing the player to click a button right from the sheet (say for a Perception check or an attack). Don't know about Astral, but in Roll20 the GM pays for some kind of subscription (monthly or yearly) at some level (Plus or Pro), and the players simply join the game. Roll20 does provide voice through the Web (WebRTC), though it's pretty common to hear of people having connection issues with voice, so a lot of gamers run Discord/Zoom/etc... No official RM support there, and while there are a few RM character sheets, they too are old and not supported. There is an end-user effort to get an updated character sheet built; look to the forums here.
The other main type is private machine hosted. Fantasy Grounds (FG) and Foundry are taking this method, where the GM hosts the game off their own machine and people join to the hosts IP. FG requires a FG client to join a game, whereas it seems Foundry is taking the standard WebBrowser and HTML route.
Fantasy Grounds is either pay monthly or one-time flat fee. Std license only allows other Std client owners to join, whereas the Ultimate license allows anyone to join (with the free FG client). Pricing is complex now due to the release of a new FG client (Unity). FG does have gaming rules built into the engine. Thus you can buy rule-sets right in FG, and have them ready to go for whatever game system you want. Need to look up a table or chart? Simply use the tools built into the client, no need to pull out your books (GM or players). Supported systems typical have fully enabled game systems, like rolling up characters right in the client and fully fledged combat trackers (tracking aspects of combat like stun, bleeding, prone, etc...). For supported games, it's quite nice; however not all games are officially supported (quite a few community resources exist). You can also simply fall over to the generic client (CoreRPG), similar to what Roll20 provides, and you play off that. Voice is not provided via the client, so people usually fall over to Discord, Zoom or something like that. RMC is sold via FG, and while the Character Sheet is functional, it's old. Rumor has it that a replacement is being tested; no idea if it's community based or official though, but I'd bet the former.
Foundry is still new; beta (release supposedly happening the end of March, 2020). You can get the client only via Patreon support currently, but it's expected to sell at ~$50-$60 with $ breaks for backers sometime near the end of the month. It seems with Foundry everything behind the scenes is HTML, and seems quite interesting, with what appears to be some solid Fog of War functionality. While it does have some gorgeous community provided character sheets (i.e. Warhammer Fantasy 4th Edition character sheet is stunning), not a lot is built for it yet.
Lot of people go by price frankly. Pay once, or pay a little each month. That said, Roll20 is pretty popular. I.e. look at the GaryCon Virtual convention (26th of March ) and of the 160+ listed VTT games, Roll20 has about 90, while Astral shows up with 2, FG with 20, and other @ 56.
