Bleh, been about a year since I played so trying to recall a few things. Servers...hmm, do some googling. Firiona Vie has an exp bonus, and you can trade things that you can't normally trade (raid gear). There are a few other servers that have specific/small differences to them, but I can't recall all the permutations or names at the moment.
FTP is certainly fine to get you going and see if you want to sub. There are restrictions on what kind of gear you can wear, and what level of merc you can hire (i.e. you can only use the worst mercs if you are free to play, this actually makes a huge difference fairly quickly). In general, I think it's well worth subbing if you are interested. That being said, there is certainly plenty of game to be had in free to play, especially if you just want to jump back in and see what's new or tool around old spots.
Start up a free to play character, do the new tutorial dungeon, which gives you a nice set of starter gear when you complete quests. It will also get you back into the swing of the game and let you know how things are now. After the tutorial dungeon, the zone they pop you out into can take you all the way to level 20 if you like. It also has a bunch of quests and rewards, and there is also something called the 'heroes tome' or 'heroes journey' that you can follow that has pretty significant loot rewards.
One other thing. The mercs are fairly worthless until about level 15 or so, at which point the tank merc becomes an unkillable god that can solo red cons. This will be true all the way to about level 60, where they start to lose some ground, and continue to get worse as you level. Between 15-60, tank mercs make the game absolutely easy mode. You can literally multi tank red mobs 5 levels above you with that merc.