Both "sith" and Jedi are "force-sensitive", the only thing that defines and contrasts the two is their philosophy on what the Force is and how one should use it.
In the now non-canon EU books Luke becomes a sith temporarily and returns to the light side with a deeper understanding that helps him become a better jedi so it is possible to walk the line but not many characters can do it successfully.
As for powers, Jedi can heal others much more effectively than sith, sith requires further use of bacta tanks and traditional medicine because their philosophy of the force is not compatible with rejuvenation, but the sith have stronger "strength" with the force in that they can move heavier objects and cast force lightning.
The Empire wasn't destroyed at the end of the Return of the Jedi, they only cut the head off the snake. There are/were still Grand Admirals and star destroyers running around and vying for control of imperial forces etc.