Let's see the pros and cons of each character.
Garrosh's Pros
1)His plan was straight forward.
2)He fought and challenged whoever he wanted personally and not sending others.
3)Created an impressive War Machine with very few resources(though Blizzard butchered every number,lore and logistics so he could manage that).
4)He has some shred of honor.
Garrosh's Cons
1)He hated a faction by the time he came to Azeroth despite not knowing a quarter of the story behind Azeroth.
2)He tried so hard to match his father's image knowing not the negative part of Grom's story.
3)He commited crimes against various neutral factions to obtain Superweapons that could end Azeroth and in a normal story that would have activated these factions against him.
4)As Varodoc said he was a fool trying to play with powers he could not comprehend (Ysaarj's Heart, the Blank Scroll)
5)He has taken stupid decisions using events to gain an advantage(Broken Front,Cataclysm) not realising the real danger(Deathwing,Arthas) or setting up events that would eventually speed up the arrival of the Legion to Azeroth.
Sylvannas Pros
1)Not sure if that is a pro but she has plans after plans after plans that cannot be explained.
2)Gained a power buff out of Metzen's Ass and might be a demigod level(Though beating an old orc and a pathetic version of the Lich King doesn't justify her hype)
3)She might have something left of the Old Sylvannas buried deep beneath the rot.
Sylvannas Cons
1)She wants absolute control like a Lich King.
2)Her Free Will was a lie after all.
3)She is naive thinking she is equal to the Jailer and not just a pawn ready to be discarded when not needed.
4)She created a War that she lost quickly just to kill as many and gain power.
5)She never explained her plan from the start.
6)Acts like Edgy Teenager when she hears the word Hope.
7As Varodoc said she is a fool trying to control Death just because she was salty she was sent to the Maw and for that she will bring Azeroth to ruins. Her Death Issues make her pathetic.
In my opinion Garrosh was the better person between the two but I would prefer neither of them. They are both responsible for the worst written stories in Warcraft, put Azeroth in danger with their issues and responsible for toxicity they caused in forums.