I suspect that High Sierra will start to tax devices that are less than 16GB, my macbook pro running high sierra is lagging a bit compared to Sierra on the same laptop, High Sierra uses a new file system for the hard drive, and my 2013 retina macbook pro is finding High Sierra hard to process, but not with Sierra...
As you will be using the laptop for a couple of years, I would suggest going for 16GB, after all you are blocked from post purchase upgrading, so if you find you might want to edit video, or do something a little complex, and you are using 8GB, you will want to throw the laptop out the window...Who knows in 2 to 10 years from now if we will have more than 16GB RAM, or if 16GB is the limit...
I really cannot understand why Apple refuses to allow post purchase upgrading or refuses to use chips that allow more than 16GB, on a mobile device, 16GB is woefully limiting..I checked on 2017 laptops and FCPX is OK, not 100%, maybe with more than 16GB it would be better...Go for 16GB....you wont be sorry...sure it is more expensive..but the pain of frustration is not worth the savings...