Wavestone
Explorer
I can only speak for myself - I use Fantasy Grounds, and is part of an ongoing campaign..I like it a lot. (In honesty, I might add that I haven't any experience with the other products..) 
One of the design ideas behind FG is that it is supposed to be lightweight - it is just a simulation of a gaming table, with character sheets, maps etc.. And not to forget the dice! (FG uses 3d graphics to show the dices rolling.. This might not be a huge feature for some, but I think it adds a lot to the feeling, seeing your d20 roll across the screen and land for the result.)
Ergo, FG doesnt do autocalculations (as I've understood that OpenRPG do?) or enforcing rules. You have a character sheet, you have to trust other people to not cheat. (might add that my group is a good one who has played for 9 months now, long time for gaming over internet.) This is a matter of taste, I think - FG is light - it is just a simulation of the table and sheets. The rules is up to the DM/group - this makes it possible to do many different rulesets, mostly you just have to make/modify a character sheet, which is doable.
Right now, some things arent really supported - like dice pools and like.. but the next patch will change that with the introduction of scripting. After that, I think more systems will be played over FG more easily.
Ultimately, I don't think the choice of "game table simulator" is not so much of "which is best?", but rather "what style suits me?"..
As for skype/ventrillo, You really forgot the option of "FG/OpenRPG with NO voip software!"
Not everyone is interested in that. I'm really glad that my group is not going to go voip, as I'm hard of hearing/borderline deaf.
Uh.. a bit verbose, perhaps.. If you have questions, just ask!

One of the design ideas behind FG is that it is supposed to be lightweight - it is just a simulation of a gaming table, with character sheets, maps etc.. And not to forget the dice! (FG uses 3d graphics to show the dices rolling.. This might not be a huge feature for some, but I think it adds a lot to the feeling, seeing your d20 roll across the screen and land for the result.)
Ergo, FG doesnt do autocalculations (as I've understood that OpenRPG do?) or enforcing rules. You have a character sheet, you have to trust other people to not cheat. (might add that my group is a good one who has played for 9 months now, long time for gaming over internet.) This is a matter of taste, I think - FG is light - it is just a simulation of the table and sheets. The rules is up to the DM/group - this makes it possible to do many different rulesets, mostly you just have to make/modify a character sheet, which is doable.
Right now, some things arent really supported - like dice pools and like.. but the next patch will change that with the introduction of scripting. After that, I think more systems will be played over FG more easily.
Ultimately, I don't think the choice of "game table simulator" is not so much of "which is best?", but rather "what style suits me?"..

As for skype/ventrillo, You really forgot the option of "FG/OpenRPG with NO voip software!"

Uh.. a bit verbose, perhaps.. If you have questions, just ask!