Round 1: FIGHT
Sidious is teh best sith evah! He blitzed teh 3 masters! He siphoned energies from Byss! He force stormed a fleet!!
But TBCH, I usually don't bother with arguments involving Sidious. Although just to throw in my 2 cents, I think he has more raw power (didn't need amulets and what not), has shown better telekenetic applications, is probably faster, and is overall just a more skilled swordsmaster.
His saber technique doesn't seem to have improved any, but he does have the force storms and such (though I will, of course, note that even his mastery of storms wasn't yet total, they were hard to control thingies and required concentration).
I'd personally think personal combat remained largely unchanged, exoteric force knowledge increased (though I bet he wishes he'd learned Dark Transfer rather than Force Storm ^^).
Though I do give DE or pre-death Sidious the win. Exar Kun is strong, but Ulic in his time was able to draw. He wasn't as overpowering as Sids was.
Yeah, but then Kun started to become an immense practitioner of sith sorcery and what not.
But here's an impressive read on Exar Kun: http://www.swtor.com/community/showthread.php?t=694388