I just curious what people think on this
me personaly would say sith is brought up with darkside thus has control over it
while a fallen jedi scumbs is more prone to exsess as s/he is suddenly imersed in the dark side
well there are a couple of ways to answer this.
First, and most obvious for people playing this game is that the Sith, unlike the Jedi, are a Race of people native to the planet Korriban. The Sith have an inherent attunement to the Dark Side of the Force as a racial trait. Their social structure is largely what paved the way to the creation of the order that would later share the same name as their race.
The Jedi on the other hand are derived from a Monastic Order that traces it's origin to Je'daii Order of Tython (Jedi being simply a shortening of the name of that original order). The Tythonian Order was dedicated to pioneering the development of Force Understanding. Though this occurred centuries before the discovery of the Sith Homeworld by the majority of the rest of the Galaxy.
So really the difference between a Sith and a Fallen Jedi amounts to whether one has been initiated into the Jedi Order, and whether one has also been initiated into the Sith Order. In the case of Dark Jedi such as Count Dooku, and Anakin Skywalker, the answer would be "There is no difference between a Fallen Jedi and a Sith." But in the case of a Sith such as Emperor Palpatine, the answer is "He was never a Jedi to begin with, so he wouldn't qualify as a Fallen Jedi."