Well, if Jedi had to combat Elites, they would use the Force. However, energy swords have dual blades, providing a more versatile use.
Assuming they were both held by a SPARTAN-II (in which they both knew how to correctly use said weapons), it would be a somewhat even fight, with the Energy Sword possibly having more of an advantage/disadvantage. This is because, while they are both created out of plasma trapped within an energy field (which is how they keep their shape), the Energy Sword is a dual-bladed weapon, this could be good and bad. Good, because the dual-blades with their tipped ends allow for a more varied attacks, but bad because this could also cause the sword to have more weight, although plasma is nearly weightless, so it just depends on the users weapon of choice. Otherwise, it should be a fair fight. EDIT: Actually, Lightsabers are made of light (hence the name). Think about it this way: Plasma can kill, it is a super-hot substance. Now you wouldn't think light could kill, would you? You don't go outside on a sunny day and drop dead. But, ever heard of that thing in France? There are thousands of mirrors pointed at ONE spot. All that light/heat can actually melt diamonds (It's a fact, they've tested this). So the fight depends on how dense the light/plasma in each weapon is.But as you can see energy swords can stop other energy forces and as you can see in a halo legends episode Thel 'Lodamee's energy sword just deflected the assault rifle bullets and bullets use energy of speed and lightsabers are energy so that the energy word could have "cut" or deformed the lightsaber and stab the jedi or sith but if its a sith he/she might win he/she will lose if the saber is one blade but if its double-bladed the sith would win