I've always ranked Skyfall above QoS because it's an overall more refined cinematic entertainment experience, but over time I've gained appreciation for QoS (which I never disliked). Skyfall is falling down on my list and Quantum is creeping up.
Skyfall wanted to be the big grand grandiose blockbuster movie like The Dark Knight. Batman's influence is felt throughout. Quantum, despite having the same budget (due to the director's meticulousness), has more of a minimalist feel to it. Skyfall has a great cast and it looks like they had a lot of fun making it and the directing is great, but it's a case of making a lot of small insignificant events seem grand. It does tackle on subjects like cyberterrorism and the symbolic bulldog representing the public attitude toward Britain's defense ministry, but the movie is not as deep as it wants to let on.
I feel like Quantum does a lot more with the development of Bond's character, a more realistic believable plot, more genuinely poignant moments, an appreciation of subtlety, the death of Mathis symbolizing the weight on Bond's chest, Camille as a non-sexualized Bond girls, attention to environmentalism being ahead of its time. There are plenty of small direction decisions I appreciate such as an emphasis for organic locations rather than green-screening, and an opera meeting for the villains instead of a typical UN hall meeting. I like how the film begins with a beautiful ocean followed by an intense chase sequence continuing just minutes after Casino Royale. A completely dissonant harsh theme song which has grated on conventional fans, but perfectly represents the feeling of frustration the movie wanted to accomplish. And with the gunbarrel placed at the end for closure. Of course, there's the writer strike, the corny oil-death scene and such, but it captures the right pathos of Bond's work.