Tabletop Simulator is less pen-and-paper focused, and it is fully 3d. You don't have character sheets, dynamic drawable maps, portraits, and other focused assets to help play - but you do have a ton of 3d tabletop pieces that are fun to play with and look at. This is more for sim-ing board games where you have many small pieces, like Axis & Allies, Monopoly, card games, or other token and board games people play together on a tabletop.
The question for me is, which does a better job showcasing a game for an audience on Youtube? While Tabletop Simulator (TTS) has a great 3d look and is very watchable as everything moves around in 3d, Fantasy Grounds (FG) is made to do the pen-and-paper job and would be ultimately less frustrating when playing and explaining things.
It feels like a battle between looks and functionality.
You can't do a live character sheet in TTS, nor can you really do a great draw-as-you-go map. They recently added whiteboard style drawing support into TTS, but it is limited and very low-resolution. You would have to make a lot of comprimises to make things work in TTS, and there may be a problem with having much at all on the 3d board resemble what you are playing with, such as genres like sci-fi, modern, and other eras are not well-represented by the included 3d assets. You do have dungeon tiles and a lot to look at, but it is centered around fantasy and there is limited expansion possibilities without being a full 3d modeler.
With FG, a lot of that is solved, but your assets are all 2d and flat. I suppose this comes down to having a great DM and explaining things as they go. To keep attention, you need great assets and a constantly changing board, which FG can do. I love the 3d look of TTS though, but I have this feeling it isn't there yet when it comes to quality pen-and-paper support.
There is another alternative, Roll20, and I want to include this in on the discussion since it is free, but it falls into the same area as Fantasy Grounds since it is also a 2d tabletop. My thoughts are more of a 2d vs. 3d tabletop comparison, so I will probably include FG and R20 on the same side of this comparison (with your preference deciding which you use).
So for game demos and streaming, which do you prefer? Do you think the market is going to a more 3d presentation, or is sticking to what works in 2d the way to go forward? It is an interesting question, and one I want to check out. More on this soon.