Roll20 has the advantage of being free. It's tricky at first, for both DM and Player, but once you pass the learning curve it's pretty solid. I've played under Foundry, and feel it's a superior setup... for the players. As I understand it from the DM, there's an initial cost to purchase, and you have to do a lot of additional setup with the system to customize it. While it may be better for players, as a DM I'm sticking with Roll20.