I have to say, Tabletop Simulator feels much more like you're sat at a table and IMO is more appropriate for casual "beer & pretzel" games, since it's hard to ignore the table! My preference for getting immersed in a game (as a viewer) is for Roll20 or Fantasy Grounds, where all you can see are the players and the dice results, with the occasional relevant handout being flashed up. This is how they do it for most Encounter Roleplay streams, and I think it's a great method.
Personally, if I ever run Alien on a VTT, I'd probably use Roll20 because there's no barrier to entry (cost, PC spec, OS, etc). Audio / video naturally needs to be run elsewhere because it is so buggy, but Discord fits the bill admirably here. I'm also keeping a close eye on Astral, because it looks like it will have even more user-friendly features, even when ignoring the map features.