Ace's death was important to make the readers feel more Akainu as a real threat setting him up as the series main villain, at least among the marines, and also to give Luffy a sense of powerlessness in face of people much stronger than him that could kill his friends without him being able to do anything about it, which was necessary as a reason to push him to make the resolution of training for 2 years to get much stronger before attempting to go to the new world again.
As for which character is better than the other, I see Sabo as much better than Ace.
Sabo has a much better design, with clothes that make him look like a Victorian Era gentleman, than Ace, who wears shorts, a terrible looking collar, and is shirtless to show his tattooed body; Just Ace's hat looks very cool.
Comparing the way they fight, Sabo's fighting style is also superior, since he doesn't just use the mera mera no mi to fight, but also uses a pipe, and especially the dragon claw technique looks very cool.
In terms of personality, Sabo is not funny like Ace, which may make him seem somehow bland, but at the same time he's also a lot cooler; I especially liked his speech about how everything has a core that you can hit to turn around any situation, which expresses his role in the story as a key character who will not be straightforward in whatever he wants to do, unlike Ace, but will always be seeking the core of the enemy to gain the upper hand.
Also, Sabo is better because he didn't subject himself to an old man's daddy fantasy.