Its often very difficult, as a mere observer, to determine exactly what makes a particular car faster than another - especially in a comparison like Mclaren vs. Red Bull. The teams are all approaching the same problem - clawing back down force lost from the ban of double diffusers - they are just approaching it in different ways. The Red Bull focuses on the tight coke bottle rear end because they are feeding their diffuser and their beam wing with air coming AROUND the side pods. The Mclaren, on the other hand, while also focusing on increasing rear downforce, are feeding their diffuser and beam wing with air coming OVER the side pods.
One of the big differences in these approaches is the impact on packaging. Mclaren have obviously had to get creative with their cooling solution, and Red Bull is having difficulties with KERS - rumored to be due to their distributed packaging of the system.
As a layman, or even a fairly knowledgeable enthusiast, it's extremely difficult to visually evaluate the two solutions. We can't see down force, unfortunately.
I think the generalization you are looking for could be something like this:
Removing impediments to airflow will often have positive impacts on downforce.
By the way... I'm not an aerodynamicist in real life, but I do play one on the interwebs.