So, first thing, the kits of 4 sticks of RAM are usually quad-channel kits (meant for workstation motherboards like X299). You just need dual-channel for Z390, so just buy what you need for the price you want. 16GB is a good spot for gaming and that's how much RAM I'd get.
As for price difference between two different 2x8GB kits, some have lower CL timings (or latency) than others. It's more desirable to have lower timings, so those kits are generally more expensive. A CL timing of around 16 or lower is good. In the product description it might read as "C16" or "CL16" somewhere in the name. Though sometimes some kits are just over-priced.
Personally, I'd go for a 2x8GB kit at whatever frequency your motherboard can handle (3200MHz is great) or for whatever price you are comfortable spending. As for CL timings I wouldn't get anything above 18, just because 15, 16, and 18 kits are so common. If, for whatever reason, you want all the RAM slots filled then just make sure you buy 2 of the exact same kits of RAM with the exact same CL timings. Do not mix different kits.
Secondly, you technically have to "overclock" most RAM to get it to it's advertised speed. However, it is super easy and not overclocking in the traditional sense, hence the quotes. You just go into the BIOS, find the RAM frequency settings and set the XMP profile to what your memory is spec'ed for (i.e. 3200MHz) in the singular drop down menu. Then save the BIOS settings and reboot and you are done. Since the RAM is rated to run at that frequency, it's pretty much guaranteed by the manufacturer to be stable at that speed.
Hope this helps!
Wonderful explanation, thanks for putting the time in