I've always felt that the Deal Ending is the canon ending for two reasons:
1. Despite what option Niko chose with Darko, he realised that Revenge wasn't really the answer and began to re-think his moral code.
Whoa, I've actually never made the connection/link with the Darko aftermath before.
Nice! Putting it this way, I agree.
revenge is more satisfying but deal is more tragic. Which one you prefer I suppose is down to the player.
Both endings were equally tragic in my opinion. The Deal ending only appeared a lot more grim due to the pacing and visual setting it presented; first Dimitri's betrayal at the docks, then the accidental killing of Roman coupled with the dramatic angle shots of Niko continuing gunning at the hitman's corpse, and the final Liberty Island scene depicted under the heavy rain, rather than Revenge's orange sunset.
But whereas the Revenge ending lacked in visual grittiness, it did had its fair share of emotional toll with it. Killing Dimitri may have proved to be epic, but the game makes it clear that the ruthless violence that just occurred may not have been all that glorifying, the use of thee empty ship halls Niko had to take before exiting the boat -the same he used to enter Liberty in the first place if I recall- accompanied with the heavy 'ringing' sound effect of the ship's haul, was at times unbearable, and you could feel the weight that goes through Niko's feelings.
Niko's relationship with Kate is based on the player's choice of whether to hang out with this person often or not at all. So for those who did hang out with Kate often, particularly to hear the conversations she has with Niko, which were increasingly full of suggestions that she 'may' just be a sign of hopeful future in Niko's life, then in her final scene, you could severely empathize with Niko's breakdown, and the ensuing gun fight is an extension of this emotional wreckage.
But most noteworthy also is that both endings followed this same idea and carry the same final line, which is the same title of the trophy/achievement earned on completion: "You won!"
Both endings mocked/debated the specifics of what it means to be truly successful, to have 'won' and 'triumphed', in this criminal underworld.
Depending on the ending, it is either Jacob or Roman who delivers the line, but Niko remains silent on both occasion. He walks away in Revenge, and in Deal he looks at Jacob with a unsatisfied/empty look on his face.
Both endings were equally tragic and shared the same philosophy on its final punch line.
The 'satisfaction' from either endings comes from how the player empathized with the precedent events; for example to the players who felt Kate was this 'barely known' person, then naturally they will empathize with the Deal ending more.
But regardless of 'who dies when and how', and of the participants of the final scene, the final line and message of the story remains the same.
Question remains though, as to whether Rockstar intended to have the choice of Deal and Revenge since the beginning, or was it down to a writer's block of simply liking both methods of outcome and didn't knew which to keep and which to scrap.
It is mostly the fact that both final levels are near identical that tips me off on this matter. As if the writers and designers liked both outcomes equally, and yet couldn't implement both at the same time.