[MENTION=6804468]leonardoraele[/MENTION]
I've seen a few good comparisons from people have used both, but I'm not sure what those links are. Try the Roll20 and FG forums. Also maybe some others will be able to direct you to them.
Here's what I've found from my research and the "consolidated" opinions I've built from what I've read. (Note, I have not used anything but FG, but I did about 20 hours of research back about April 2015 before deciding.) Also, here's my review of FG;
http://www.enworld.org/forum/showpost.php?p=6750807
Though Roll20 is "free", it really isn't. Not if you want any of what it considers advanced options and what FG and the other "paid" apps consider core or basic functionality.
I've heard multiple users of both FG & R20 comment that from a player perspective, they are very similar, though maybe R20 is easier to use. But, from a GM perspective, FG is much easier to use for creating campaigns and preparing for game sessions.
If you want to do more with FG or R20, both have tools to do so. Macros in R20, xml coding in FG. For FG, no coding is needed to play or GM. It's only if you want to do something like change the interface icons, add a second chat window, make an integrated database extension for all the shops and businesses in your campaign (which a community developer has done and has published for free use).
Most complaints about FG come in three forms:
1) Cost:
- comparing the free R20 membership versus the FG license. Not a valid comparison because you are not comparing even remotely similar functions.
- complaints about the cost of the license 5E material. Which is completely optional, and really missing the point. If you want to load in your 5E books yourself, their is a community developed tool that helps you bulk load your own PDFs into FG with the full FG drag & drop type of functionality. Also note that all the mechanics of numerous rule systems (including 3, 3.5, 4 & 5E) are all included free with the base license. And, as of this month, FG now has license to include for free the 5E basic rules from WotC. So you actually get the basic rules for 5E and not just the game mechanics.
- Both R20 and FG are about $10/month if you want a subscription model and competitive features
2) Fog of War / Dynamic Lighting:
- FG uses a simple mask over images that the GM removes in parts (either with rectangles or freehand shapes)
- R20 has the ability for defining lighting sources and obstacles so that the map reveals itself as the light sources (players) move about
3) Mapping tools. FG has very limited tools for marking up maps. They work OK during play, but are bad if you are for some reason trying to actually make a map. You will need to make your maps in another application and just use them in FG for combat (if you even use maps). Do note, that FG includes 40+ generic maps for free for when you need a non-specific battle map such as an outdoor location, a city street or a cave passage.
Some of what drove me to FG:
1) 5E licensed material. I'm at a point in life where my time is worth money to me. I would rather pay for the licensed material than spend the time either creating my own or inputting information (feats, items, etc) when I need/want them.
2) Longevity. FG has been around and active for many years. It is unlikely that the program or company (SmiteWorks) is likely to go under or otherwise become obsolete in the next 5 years.
3) Active development/enhancements/support & user community. FG's community has amazed me from the start. People are incredibly helpful. Tech support is thorough and responsive. FG continues to roll out new functions and is making major long term platform investments (move to Unity, Table Top Connect merger...)
4) Locally installed application. FG is a piece of software that is downloaded, installed and run on my hardware. This means I do not have to be online. This means that if FG goes out of business, I have the software and all my content forever. It means I don't have to worry if they get hacked or targeted by a DOS attack. It means I don't have to worry about their hardware performance.