Um... you can find helpful groups of people and build long-lasting friendships in both LS and FC, so don't worry so much about which one will allow you to find people to do content with. Either or will allow you to pull together groups to help with low level dungeons, nm primals, as well as whatever content you want to pursue in the future once you hit 50. They're both useful ways of networking.

Basically, the similarities are that they both provide a chat channel, and they both can be dedicated to certain content or playstyles. (there's both endgame and casual fc's, and there's ls's dedicated to certain content like hunts, raiding or primal farming, etc. as well as just being used as a casual-based chat channel. )

The differences are, that your fc will be your main group (like someone else said, like your in-game family sort of) and will give you access to in-game content that you wouldn't have otherwise. You can take as long as you need to find the right fc for you, but think about not only where you are in-game right now, but what your long-term goals are, and search for a group that have the same goals and over-all playstyle as you do and you should be good. An LS is more or less an extension of your in-game community, and a lot of times ends up being a more specific group than an fc though sometimes more generalized too. For instance, I have a hunt LS, an NA LS (I play on an EU server so it serves as a gathering place for other players who have similar playtimes to me), an LS of old friends from XI, a PVP LS, and just an ls for friends, among others. They don't have to be connected to your fc though some of them probably will be, they're really just a gathering place for groups of friends or people with similar interests that is separate from your main group.

Anyhow, that's just a little run-down on the differences and similarities between them, but don't feel rushed into joining one, likerightnow!!! if you think that a certain fc or ls might not be for you. Also, if you do join an fc and find that you don't think you're a good fit there, don't be afraid to leave it and find another one that suits you better because it might save you some future headaches. People do tend to put more into building up an fc because of the content that fc's are linked with (like housing, gardening and a number of other things), so you'll want an fc that you're comfortable with for the long term. But really, just play at your own pace, and go with what you feel suits you.