Alright. I'm ready to make the comparison.
First, let me state what I believe makes up a powerful Jedi fighter.
Training Time
Experience
Lightsaber Style
Knowledge of the Force
Mastery of a Lightsaber Form
Understanding of how to best utilize said form
Better Form for dueling
Dueling
Dooku had almost all of these over Mace the first time they fought, but this will probably not be so this time. Let me show you why.
Now, let us examen Training Time.
As of the first time they fought, Dooku had been training for 67 years. Mace had been training for 37 years. Dooku has almost double Mace's training time at this point. This is assuming, of course, that Dooku defeated Mace at the last possible moment before leaving the Order. It is more likely that they fought earlier than that, so let us go with five years before Dooku left the Order. Let me note that this is still very favorable for Dooku.
That means that the last time Dooku defeated Mace, Dooku was 63 and Mace was 33. Mace has just three years of from being doubled in training time.
Now, if we look at them as of ROTS, Dooku is 83 and Mace is 53. Dooku of course still has 30 years over Mace, but now their ages are closer proportionally.
In their first fight, Dooku had about 190% more training time. Now he has about 157% more. He still has an advantage here, but by how much?
Next, lets look at something most of you probably never considered.
Dooku believed in the Potentium. That is, for those who don't properly understand the Potentium view, he followed both the Lightside of the Force and the Dark. Potentium followers believe this is okay as long as they do not favor one side over the other. This was the reason Dooku, nor any of his students, were ever offered a seat on the Council.
So, seeing as Dooku already followed the Darkside to an extent, he didn't learn a lot under Sidious, or at least not as much as he would have normally or as much as you guys seem to believe.
Being a Jedi Master, he would have access to all of the Dark Side knowledge that the Temple held, which was a lot. They held at least one Sith Holocron, too.
Since he already knew a lot of what Sidious told him, he wouldn't have advanced as fast in this time. Lets say he only advanced a two thirds as fast as he normally would have. That gives him 73 year of training by their next fight.
That 190% that he had before is now just 138%. Just 38% more experince.
Remember, Dooku used the Dark Side even as a Jedi.
Now, lets look at experince. This is very similar to training time, except that we will weight actual events rather than regular training.
First, Dooku has done the following:
Killed a Jedi Master
Defeated a Master and Knight
Held off Yoda
Led the attack on Gilidrann
Trained six students
Mace has:
Stopped 14 Gank Killers
Killed Uda-Khalid
Was always on the top of his class
Trained two Padawans
Created his own lightsaber form
Killed Jango Fett
Led the Battle of Dantooine
Defeated a Jedi Master even with serious injuries
Survived a fist fight against Kar Vastor
Defeated Sidious
Mace seems to be a fair bit more experince by ROTS. Also, considering that Mace defeated Sidious without being extremely weak after(judging from facial expressions) he was a fair bit more powerful than Sidious.
Lightsaber Style: Dooku had a major advantage in lightsaber style when they first fought. Dooku will not have such an advantage this time.
Although it is not known exactly when Mace developed Vapaad, it was still very new when he lost to Dooku.
When Mace was 14, he still had not built a lightsaber. We know that Mace was a powerful form five user and that he had to learn Juyo before he could develop Vapaad.
If he learned both Juyo and Djem So in the time it took Obi-wan to master form three, the first time he and Dooku fought Vapaad didn't exist.
Mace also had to learn form 1.
So, Mace might have mastered Djem So and had some knowledge of form 1(cant remember the name) and Juyo, but Vapaad still didn't exist. In this fight, Mace will have finished learning Vapaad but he will also have Vapaad on his side.
Knowledge of the Force: Dooku has greater force attack skills than Mace even in ROTS. There is a major difference now though.
The first time they fought, Mace didn't have Vapaad. Vapaad was what allows Mace to withstand Sidious' force attacks. He couldn't defend as well against Dooku's force attacks before beause he didn't have Vapaad, but now he does. Sidious was more tired after his fight with Mace than Mace was(judging from expressions) showing that it uses less energy for Mace to defend with the force than it does for someone else to attack him with it.
Mastery of a Lightsaber Form: The first time they fought, Dooku has been learning Makashi for over forty years. Mace had learned some of form 1, possibly mastered Sjem So, and learned some of Juyo. Those other forms are weaker than Makashi for saber vs. saber but the same doesn't not seem so for Vapaad. Mace has also shown himself to be a very skilled duelist, adding hand and foot techniques to his lightsaber skills.
Understanding how to best utilize said lightsaber form: Dooku had an advantage here that he will not have against Mace. Mace didn't know Vapaad nor had he been in as many conflicts that required use of a saber. By the time they rematch, Mace had discovered how to fully tap Vapaad without falling to the Darkside(his time on Harun Kal had a lot to do with this.).
Better form for dueling: The first time, Dooku had an advantage in that his form was better for dueling than Djem So. It seems that Vapaad's use of a lightsaber along with it's special focus of mind would give it an advantage, if only a minor one.
Dueling Skill: It is unknown how good Dooku was at dueling as of the last time they fought, such as being able to fight without a saber and incorporate that into his form, but we do know Mace is very good at it as of ROTS. He was able to fight effectively against Kar Vaster. It seems to me the Mace is better at dueling, but there isn't enough evidence at this time prove it to anyone.