Almost certainly in a fight between tool and mok, whichever Se thought would make a better story would win
That said if you consider the textual evidence, we can't know, but it's most likely Mok would win, from the following points:
1. Tool was unmatched among the imass, thats a fact, but how many imass are there in reality? there were 7 clans left, whose numbers as they stand are maximum 60,000, and in most cases significantly less, assuming attrition, lets say that we started off with 100,000 in each clan, then add a few more for good measure = a million imass, so Tool is the best swordsman out of a million imass.
2. The imass in general fought against jaghut, not particularly against each other, that requires swordplay to defeat a supremely powerful opponent who requires a large number of powerful blows to disable, see the seemingly indestructible nature of some of the jaghut, with several others to help, hence the masisve power of the two handed imass blades, the focus is clearly not on one on one duels.
3. Mok is a seguleh, a culture which spends their whole life fighting other people with sharp pointy objects, to the point where it is there sole aim in life. A single 14 year old seguleh senu is a swordsman almost without equal among the vast majority of people, it's not unreasonable to suggest that senu is one of the best out of a million people, and mok is one of the best out of culture whose average member is around that level, who specialises in fighting duels, he's got a definite advantage in a one on one duel with his weapon choice as well.
4. Tool is almost certainly faster, but as has been stated he loses some other abilities, but I think he probably is stronger than he was in a one on one duel, he probably can't take on armies as easily, but he's got an army of his own to back him up. That said he has acknowledged Mok as his superior after there fight in MOI, and earlier on expressed his doubt about beating him, the imass time has passed the masses of humanity have managed to produce someone who can beat the imass first sword.
Tool is good, very good but Mok is freakishly good and I'd bet money on him winning again. That said if I had to choose between Tool and Mok as someone to defend my life... I'd choose tool.