The Jedi Order is the greatest Force-user organisation because they act as a political/diplomatic force. By being an integrated part of a galactic political power, they have the scope and influence needed to expand both their understanding of the Force and their ranks. Being able to recruit across the whole known space is a big deal, especially considering how scarce potential Force-users are.
The Sith are the Jedi Order's shadow: the first Sith was a Jedi who rebelled against the order because he wanted to explore the dark side, something forbidden by the Jedi Order. The Sith became a faction by itself when more Jedi joined them, attracted by the dark side.
Both Jedi and Sith are curious by nature and want to develop their use of the Force. The difference is that Jedi apply a kind of Vulcan code - valuing peace over conflict and rational thinking above all - while Sith aren't afraid of either the chaos or the dark side.
Anyway, the Sith are mostly Force-users that could be Jedi if they were willing to restrain from emotional reaction in order to stay away from the dark side. In other words, any Force-user that see either emotions or the dark side (which are quite similar things in fact) as assets will become a Sith if they aren't already.
The fact that both always try to rule over the galaxy and that any Force-user trained by one side can turn to the other depending on their predispositions, they are both sides of the same coin - that coin being either the Force itelf or the current galactic order.