Changing my vote to Fisto now that I give this a more detailed analysis. Both of them use forms that are weak in single combat (shien and shii cho respectively), but Fisto has better dueling feats than Koon (Handily beating Post AOTC Obi Wan (a master of Ataru and Soresu (although not to the extent he was in ROTS), and whom honestly did better against Dooku than Ventress ever did at any point), Grievous (a master of all forms in terms of just technique, who uses Jar'Kai and Makashi (since he has to duel jedi and was trained by Dooku), and this is especially impressive given that Makashi is often shown to be kryptonite to Shii Cho) and briefly holding his own against Palpatine (the degree in which he does varies somewhat by source tho), at least to my judgement (by comparison, Plo lost against Savage Opress (who is weaker than Maul, meaning he probably would've been dissapointed with Koon, much like Jinn), and only debatably defeated Ventress (and even if he did, Form V has frequently been shown to counter Makashi and Jar'Kai, which Ventress utilizes (it's not the first time that sort of thing would've happened either, a la Anakin and Opress)
Physicals should also go to Fisto. They're about even in strength, Fisto is a good chunk faster (Pre-CW Obi Wan could barely see him) than Plo, and Fisto's ability to sense the pheremones of others via his tentacles gives him slight precog along with his force precog, which IMO gives him a slight edge with even higher combat speed (which might be how he blocked Sidious' first few attacks).
I think Fisto should win via semi-blitz. His greater speed along with his ferocious and unpredictable pattern gives him an edge imo, and while Koon does have greater precision with the force, he'd need a second to gather his energy to actually overpower Fisto, which won't happen when Fisto is barraging him with what can be described as "Poor Man's Windu" attacks.