First of all, the advice OP got until now is sound, SSD takes precedence over extra RAM, but I'd like to contribute with some extra info so people who make a RAM buying decision know what to expect.
This is how typical memory usage looks like on my HTPC (pic bellow) . It was built as a Plex & file server, but also with a backup role for Photoshop work. Win 10 is pretty decent at using excess memory for caching purposes, and with stable memory usage it can grow that cache to really surprising amounts. Considering at least some of that cache is comprised of media content stored on mechanical storage, one can guess there are some tangible benefits to be had from it.
In contrast with the 10GB of cache my HTPC is using, here's how my work machine looks like right now from a memory perspective:
Different usage pattern, different memory allocation. I should also mention the HTPC sits around 500-600MB of cache right after a system restart.
As long as memory is cheap to acquire and does not affect budget for other components -
especially SSD, it can be a good idea to overshoot RAM amount by a factor as big as 2:1. In my case the decision was kind of a no brainer, since I bought when RAM was cheap and I also needed to make sure I could some day run memory intensive apps on that system. Normally, running a Plex server requires little RAM, could have gone as low as 4GB.