In my opinion it COULD be a good fight, but only if one little key detail were decided in a certain way.
Do light-sabers disintegrate matter or melt it?
If they disintegrate matter, then the Spartan is going to die. They cannot use any of the weapons against the Sith at this point and the Sith can take their time picking them apart (potentially limb from limb).
If they simply melt matter, then the projectile weapons the Spartans primarily use will be very, very effective. Especially since the Sith will be used to laser blasts instead of good ol' lead bullets. The Sith will probably end up attempting to block the bullet with his/her lightsaber and end up with molten slag in the face. If, by some random chance, the Sith decides to attempt to use the force to block the bullets then he's going to have to be REALLY good at the force to do so. Stopping one little bitty bullet would be a moderate challenge to a basic force user. Mainly because of the size of the bullet and how fast it is going.