In examining the 2nd Edition Codex: Angels of Death, we are looking for differences to enjoy.
In the 5th Ed. codex, it reads, "In evil cunning, the Warmaster offered Sanguinius one final chance to renounce the Emperor, to join with Horus' inevitable victory - yet the Primarch of the Blood Angels held true, and refused."
In the 2nd Ed. codex, we instead are treated to, "Although Horus offered him a throne by his side and the pick of human worlds to rule, Sanguinius refused, even though he knew such a refusal would seal his own doom."
Let's actually examine this:
The Emperor is on his way and I am about to win. Sanguinius is here, let's offer him something that is a pittance compared to being the undisputed ruler of the universe. His additional help will make sure I can defeat the Emperor. Horus has nothing to lose by making an offer, essentially. Horus has a lot to gain if Sanguinius would turn to the forces of Chaos.
Does Sanguinius not already have a privileged place as one of the primarchs at the side of the Emperor? Does he not already have a homeworld with two moons also? However, with his prescience it really is a question of life in heresy or death in honour. Could it also have been a gamble that he could hold off Horus long enough for the Emperor to enter Horus' sanctum to prevent his death? The 5th Ed. version has the Emperor immediately arriving after Sanguinius' death. The 2nd leaves no mention of how long it took the Emperor to arrive.
Knowing that the Emperor is on board the ship with you would have to be incredibly fortifying and encouraging before the biggest test of your life, I would have to say that Horus should have given a much improved offer to provide a greater temptation. The great Warmaster and tactical genius failed at the most critical time twice and ended up paying for it. Of course, letting his shields down was a failure made from personal character. Not swaying Sanguinius to his side shows his limitations.