Yuan Shu's glaring fault was that he was an A-hole.
Yuan Shao's glaring fault was his indecisiveness and lack of listening skills.
Yuan Shu, although an A-hole, might very well have been a better ruler than his half brother, seeing from things like how he was forced to build his army up twice and actually had huge numbers (although not as big as Shao's, probably due to his geographical position, in comparison to Shao, who was surrounded by easily conquerable forces), but then again, he picked Liu Bei, Tao Qian, and Gongsun Zan, of all forces to ally with, while Yuan Shao sided with Cao Cao, Zhang Miao, and that other guy (Shao probably could have decimated Y. Shu's army just with Cao Cao, even if Y. Shu had Liu Bei, Tao, and Zan all helping him.
And, another thing about the two Yuan's is that Shao was the older, I think, and therefore, would have been favored by their parents, and a majority of the fortune of their family probably would have been handed over to Shao.
And also, he did take a better route on which side to take when trying to decide between siding with Liu Biao and Yuan Shao when they were attacking Sun Jian after finding the Imperial Seal. (Sun Jian was the man, and in my view, if he had lived longer, he should have been the "Good guy" of the story, as I never saw Sun Jian do any thing that was even halfway sinister or evil, and he quelled some other rebellions after the Yello Turbans, and even earned the title, "The Tiger of Jiang Dong"...That's all there is to it.)
But, that's getting off the subject...He was also trusted enough by the Sun's for Sun Ce to flee to him when Sun Jian was killed. Although, Shu made a very evil mistake, when he decided to use Sun Ce's inherited Imperial Seal, and use it to proclaim himself Emperor; That lead to a mini-coalition, lead by Cao Cao, but Y. Shu didn't let pride get in the way of his livelyhood, and humbled himself, by trying to flee to Shao. But, he was stopped by Liu Bei, and forced back into his castle, and his castle actually lasted quite a while, as before Cao and Liu could get into the castle, he starved to death.
But, at least Yuan Shu listened to his advisors (although, he also followed bad advice, too.), and at least when he died, it was because he was surrounded, outnumbered, and therefore, afraid to try to break through the lines of his enemies, but when Yuan Shao fought Cao Cao at Guan Du, he had actually outnumbered Cao Cao at the start, but was outmaneuvered, and then, killed.
So, I would have to say that the Yuan's are just about tied, if Shu isn't a little more competent.
According to Yuan Shao's biography on kongming.net, he and Yuan Shu did not get along, ever since they were children, but I guess they respected each other enough for Yuan Shu to trust Shao to protect him, when he tried to run from Liu Bei and Cao Cao, (and hypothetically, vice-versa, probably.)...
But, that's not the point of the topic, either, so, I'll go ahead and answer. I would have to say that the Yuan's are just about tied, to me, as Shu's big fault was being an A-hole, but out of every one billion people in the world, there are probably around 650,000,000 A-holes
, so it's not like he was the only one.