Niko accepts the deal. Roman dies at his wedding. The mission A Revenger's Tragedy follows.
What it costs: You lose his cousin, the only character who saw something good in Niko throughout.
Two endings. Someone dies in both.
Near the end, Niko chooses between Deal and Revenge. The choice is irreversible, and someone close to him dies in either case.
The ending is described below. Players who have not finished the game may wish to skip it.
Both outcomes side by side
Niko pursues revenge. Kate McReary dies at the wedding. The mission Out of Commission follows.
What it costs: You lose Kate. Roman survives, and later games treat this as the outcome that occurred.
The financial reward is identical in both cases: 250,000 dollars. The choice is therefore not about money, but whom you are prepared to lose.
Which outcome is considered canon
The 2009 expansions suggest that the Revenge outcome applies. Rockstar has never called it official canon, so this is an interpretation, not confirmation.
The ending is not the game's only choice
During the story, you also decide the fate of three other characters. The game never suggests a right answer, deliberately.