For my applications, quantity is better. Since I do CAD work in addition to 3D scanning with only occasional gaming, I need the capacity.
While I am 3D scanning, I can use in upwards of 30GB of RAM (or more) in one session. CAD work may be just as intensive in the first stages of processing those files. However, I wouldn't consider that "typical" use for someone.
For what you describe, I doubt you will see much of a performance hit unless you are benchmarking and being super picky about the scores. My immediate answer for you is quantity over speed, but you need to test and work with both configurations yourself.
I don't think I saw anyone mention that under-clocked RAM may be unstable, in some circumstances. After you get the new setup booting with additional RAM, do some stress tests with Memtest86 and Prime95. If those are unstable, play with the memory clocks and timings a bit to find a stable zone. (Toying with memory speeds and timings can get complicated quick, btw. Learn what timings mean first before you adjust them as clock speed isn't everything.)