I don't think many people understand how a lightsaber works at all. Anyway straight to the point an energy sword would be useless against a jedi/sith. Why you say?
1st. a lightsabers design is unique, it can be designed for many various techniques, both defensive and offensive. An energy sword however has NO customization. It is designed for the sole purpose of powerful and often lethal strikes. However note the fact that this doesnt mean its made for duelling.
2nd. a jedi/sith despite having less strength than an elite wielding an energy sword, would more than likely make up for this defficency with speed, along with agility. You see those powerful sword strokes don't mean anything if you can quickly parry them/dodge them then counter attack.
*just imagine a claymore (greatsword) against a rapier, same basic principle, u got might vs speed, it doesnt take a genius to figure out whose going to win*
Bottom line: A lightsaber would destroy someone using an energy sword any day any time. Energy swords aren't made for duelling, its made to inflict fatal blows in a single strike. But if its blocked/countered by another skilled swordsmen wielding a lightsaber, its over.
TL,DR: slow, unwieldly, but powerful vs quick,agile and powerful. Try and guess who wins?