...some of whom happen to be the same people.
That's pretty irrelevant given the direction of the shows being so different. Basically, you'd never get an "In the Pale Moonlight" with the TNG writers/staff/producers/etc. Their direction was largely episodic and far more archetypal.
I guess I'll go over this again.
1. Gowron was an outsider. He had no political ties or ins with the High Council. Killing K'mpec would surely have been in Duras' favor given his political power, which his family was established as having a great amount of. Enough so that any split against him could in turn cause a great civil war, which the Empire did not want.
2. Normally, arbitration fell to a member of the High Council. A large part of the council was loyal to Duras, which is why K'mpec said they couldn't be trusted. Because of that, he appointed Picard, which was an extremely unorthodox move that could not be predicted. It's extremely likely the assassin was not counting on that, and was thinking the council would arbitrate. Given Duras' position within the council, it is likely that he would have been the successor. Certainly, the council would not oppose Duras since they feared the civil war.
3. Duras was shown to have a pattern of cowardice and dishonorable behavior. He tried to have Kurn
and Picard assassinated, but he didn't try to carry it out himself. He also tried to kill Gowron with a suicide bomber. He didn't want to risk his life in a confrontation, so he used others to achieve his goals. Unless it came down to killing or threatening more defenseless women. He killed K'ehleyr and threatened to kill Kahlest just to cover up his own dishonor. With a track record of such behavior, poisoning someone doesn't seem out of the question.
4. Despite the reasons for his potential dishonor, he still engages in the crimes of his father. He had help from the Romulans in the form of technology for his suicide bomber. Allying with the Romulans was obviously perceived by many Klingons as one of the worst things that could be done. And Duras engaged in such alliances purely to gain an edge in grabbing power.
5. Duras (or one of his operatives) had the proximity to K'mpec to deliver a sustained poison. Gowron did not.
6. Duras constantly challenged K'mpec's authority and did not respect his decisions. The latter could also be said of Gowron though.
7. If Gowron was the next to lead the people with no more opposition, it is likely that there would be those out to discredit him. Surely someone followed up on the investigation of the poisoning, and they likely found it to be done by Duras, which they would keep quiet about given how his family still had power. If it were Gowron, it would have been known very quickly.
These don't completely rule out Gowron, but they very heavily point towards Duras. Given these, the admission of the writer himself, and the predictable nature of TNG, it's a really safe bet to say it was Duras who did it.