Should I upgrade to 16GB of faster RAM, or does speed matter more than capacity?

Upgrade to 16GB of faster RAM — capacity matters more than speed. Going from 8GB to 16GB is a huge performance boost for everyday multitasking, while the difference between 2133MHz and 3200MHz is barely noticeable (2-5% in benchmarks). 8GB is the bare minimum now, so get the 16GB kit. More RAM is almost always a better experience.