Galactic Empire would flatten the Sith Empire in any space engagements, it was revealed that at the height of their power there were 20,000 Imperial I class SDs out there, that isn't even counting the rest! Then you have an SSD for every sector.
Then you have the most powerful ship sqaudron of all time: Death Squadron, Vader's personal fleet of ships, led by the Executor SSD with six other SDs, like the Avenger, their combined power took out a fleet of 40, that's right 40 Rebel cruisers, including two Mon Calamari Star Dreadnoughts.
It seems using the information we have and simple maths, that there were around thirty Super Star Destroyers before the defeat of Endor.
The biggest game changer is the Empire's size, which also means a far far superior war industry churning out better ships at a faster rate than the Sith Empire could, add on top of that, the Battle Meditation that the Emperor used on a galactic scale and you have the Sith Empire losing battles left, right and centre.
Then you have an army of such a larger size than the Sith Empire, that it isn't even worth bothering to mention that the manpower pool goes in the Empire's favour.
The Intelligence branch of the GE is also much much better than the SE's.
So the GE has far more men, far more ships of better design, far better intelligence, better commanders overall, more space to not care about, more industry and the most powerful Sith Lord of all time influencing every battle.
Here is the one advantage the Sith Empire has: better ground troopers, by that I mean the Sith, with more powerful commanders, etc.... in the Sith Lords who would lead them.
Let's not even go there with a one-on-one battle, Sidious would school him in lightsaber combat, has the most powerful Force Lightning ever and can become the most powerful and devastating Sith being by an even higher degree than before if he does somehow die.
I don't mean to be snappish, but there seems to be a lot of speculation here due to not having info regarding the SE.
1. Sidious's ships get zero tech advantage per Kaggath rules. They only get to be better tech-wise if they were awesome in their time (they were awesome for their time, but so were the SE's ships)
2. GE's won't have more ships then the SE. The SE has been building for a thousand years I think? They're be making ships just as, if not faster, then the GE's ships.
I just can't think of anything that would suggest that Sidious's fleet would be bigger then Vitiate's. Heck, I would think Vitiate's fleet would be bigger since they'd been growing for a thousand years.