5 is superior imo, it's more simple to understand but has a lot to master. It's more visually appealing too, Civ 6 throws so much extra crap at you.
5 is more depth from simple mechanics. 6 is adding mechanics to try and make depth.
I'd recommend 5 for sure, though 2K are kind of scummy and put 6 on discounts to try and force players into that and no longer really put Civ 5 on sale cause they know people would rather buy that.