I suppose in part it might also depend on whether you only wanted to go by canonical data, or wanted to include other possibilities as well. Going by what FASA wrote, there had already been one major war between the UFP and the Klingons (the Four Years War) by the time of TOS, and another war was just breaking out when the Organians imposed their terms on both sides. It's implied that the two powers are roughly equal in strategic terms, so in the event of a future war it's not inconceivable the Klingons could have gotten an edge.
It's also up for grabs what sort of covert help either side might have gotten, if any. The Romulans might have backed the Klingons once the war started to go in their favor, as Sela's forces supported the Duras family during the Klingon civil war. The Enterprise-C is consistently referred to as an "old" design by military standards, and Yar says that the Federation has lost half the fleet to the Klingons by the alternate 2366. Perhaps the Klingons would have been more willing to do what the Jem'Hadar often did and make kamikaze attacks a part of their tactics?