My understanding is that if the DM has the 'unlimited' license, or whatever it's called, FG is completely free to the players. I've never run a game in FG before, but I have played one, and it worked pretty well. My only reason for never attempting to run in it was the price (it's base price is higher) and that if you want to take full advantage of the features for the game, you have to buy the book you probably already own a second time through Fantasy Grounds. That's a pretty steep lead in point. However, I will say it does save you some work, so it could end up being a pro rather than a con.
I currently run games through Roll20 fairly regularly. It was fairly easy for me to get everything working the way I wanted in the free version. I used it so much I paid for the upgrades. I use the simple sheet in my home tables, and I was able to mostly automate starship combat and full automate everything my NPC's do with a few minutes of work and some macros.
I have not considered purchasing the in-game compendiums for Roll20, as I really dislike the official sheet and I am loathe to buy yet another copy of a book I've already bought (twice or more, in a few cases).
Though, again, my understanding is that if you buy the compendiums from them, it could save you some work, so that may be worth the money for you.
Thanks @pithica42! Yes, the monthly fee is an issue for us. We will continue our regular game at the "real" table and SF via a VTT is just an addition, so we might end to play it not thaaat frequently. So we might end up with Roll20 again.
(The screenshots of SF games via FG look great though!)