Dueling Abilities:
Plo Koon
Plo Koon is an extremly skilled duelist and is a master of the 5th form of lightsaber combat.
A skilled and adept warrior, he fought many battles during his career. Plo hailed from a long Kel Dor Jedi dynasty, including an uncle and Plo Koon’s own niece, Sha Koon, who fought in the war, as well. Plo was a master of the powerful Form V lightsaber combat.
- Source: Star Wars: Galaxy Guide 15: Attack of the Clones
It was noted he'd be a challenge for any Jedi.
With advanced telekinesis abilities and a mastery of the powerful Djem So Lightsaber form, Jedi Master Plo Koon would prove a challenge for any Jedi.
- Source: Star Wars: The Clone Wars: Adventures
According to the Legendary Jedi Master Qui-Gon Jinn, Plo Koon was an expert duelist.
"He's an expert swordsman and a hard taskmaster."
-Source: Qui-Gon Jinn
Plo Koon easily disarms Ex Jedi Aurra Sing, the same person who gave Legendary Jedi Master Qui-Gon Jinn brief trouble.
Plo Koon has held his own against power duelist Savage Oppress, before being defeated via mask removal. He even manages to land a hit on him.
With a broken arm Plo Koon holds his own against Asajj Ventress. Ventress has been known to give Anakin Skywalker and Obi-Wan Kenobi trouble.
Ki-Adi Mundi
Ki-Adi Mundi was considered valuable when he was Jedi Knight
I would urge you to consider Ki-Adi-Mundi. Though he is still but a Jedi Knight, he previously stood in for Giiett and has demonstrated exceptional judgment and valor.
-Source: Jedi Council: Acts of War 4
Yoda had thought that Mundi was capable of defeating Sharad Hett
Of all living Jedi, have you only the skills to defeat Sharad Hett.
-Source: Outlander
Ki-Adi Mundi briefly dueled Asajj Ventress, before she was forced to flee.
Most impressively, Ki-Adi Mundi contends with the OP version of Grievous for the longest time in a team comprised of Ki-Adi Mundi, Shaak Ti, Aayla Secura and K'kruhk.
This is incredibly impressive, because not only was Ki-Adi Mundi exhausted from previous battles, he was at a huge form disadvantage. As a master of Ataru, he'd be less effective against multiple opponents, or in this case multiple blades.
https://www.youtube.com/watch?v=WIj7gIDFDe4
Both are incredibly close as duelists and I can't justify giving an edge to either combatant through pure skill, however there is a clear difference in how they fight. Similar to Qui-Gon Jinn, Ki-Adi Mundi usually rushes into battles unprepared and without caution, even if he's on the losing end of a fight. In contrast, Plo Koon is a much more calculated and tactical fighter. For this reason, in a pure blade to blade contest, I'd support Plo Koon.
Like Rancisis, Koon had a keen mind for military strategy.
- Source: Star Wars: Cloak of Deception
Dueling Abilities = Plo Koon
Force Abilities:
Plo Koon
Plo Koon was among the most powerful of his fellow Jedi.
Plo Koon is a member of the Jedi High Council and a Jedi General in the Clone Wars. Koon is one of the most powerful Jedi ever, with awesome fighting ability, strong telekinetic powers and superb piloting skills.
- Source: Star Wars: Character Encyclopedia
He's performed notable feats such as disabling droids with telekinesis, pushes large structures and collapsing caves.
Ki-Adi Mundi
Ki-Adi Mundi was also impressive with his force abilities, able to knock down several sand people, create craters in the ground, lift huge skiffs and even heal minor injuries with the force.
While I do think Plo Koon has more raw power, I think Mundi's use of the force is more of the combative force user. Plo is willing to use his abilities, just not as much as Mundi. For this reason I see no edge given to either.
Force Abilities = equal
Physical Abilities:
Plo Koon
Kel Dors have a naturally weaker constitution than other species and this is a weakness that has hampered Koon numerous times.
Despite this, he's done impressive things with hand to hand skill.
Still, Ki-Adi Mundi has a huge advantage. He's highly durable, strong and skilled with his hand to hand combat.
He's kicked into metal pillars by Grievous and gets up uneffected.
A hit from Mundi could be devastating for Koon.
Physical Abilities = Ki-Adi Mundi
Conclusion:
I can see this going two ways.
1st way: The duel would last for an incredibly long time, however it would be Mundi who'd remain fresh and Plo would be wore out, not being able to contend with Mundi's superior physical abilities. Eventually Plo leaves an opening that Mundi would exploit.
2nd way: The duel would last a while, though Plo knows he cannot win in a war of attrition. Using Mundi's straightforward fighting style against him, Plo manipulates Mundi into making an opening.
I see these two scenarios as being equally as likely, therefore I see this duel going either way. I see it as a draw.
Winner = draw