Installing RAM - 27 inch late 2013 iMac

My iMac is running slow and I am hoping to get a few more years out of it. I figure I have two options:

1. add memory
2. replace my current hard drive with one that is solid state.

So I am tackling the memory option first. My iMac has four memory slots with two in use. The two in use each have an 8GB module. What can I add in the empty slots? 4GB or 8GB modules? Also, can someone provide a link to a company so this newbie can purchase the correct modules? For example:

https://www.amazon.com/Crucial-Single-PC3-12800-Unbuffered-204-Pin/dp/B006YG8X9Y?th=1

I'd suggest you look at your RAM usage in Activity Monitor first. 16 Gb is plenty for most tasks. If you're seeing low memory pressure (Green on the graph), then you might be better off with the SSD first.

Here's an example of very low RAM usage: tiny amount of Green; no Swap, no Compressed memory. If you've been running for a long time, you can expect some swap.

Here's a similar setup using 8Gb of RAM, which is coping, but could benefit from more RAM:

Pressure is still green, but higher. It's not Yellow or Red, though; but it's having to compress some memory to cope.

Don't forget that MacOS tries to use as much memory as possible, so simply using lots of RAM is not an indicator that you need more.

An SSD will be a much more noticeable improvement, with app launches, document loading and other disk-related activities all being orders of magnitude faster.