They're both important. Certain fish handle certain water types better, there is no right answer. Some like tetras like very soft water, others like cichlids like harder water (I am referring to GH
like you said is important in terms of pH. If it is too low, the pH in your tank will swing around and cause fish to die. On the other hand if it is too high you will not be able to adjust your pH. Usually if you have a high kH
your pH will be high too. Again, fish like platies or cichlid prefer this water.
It really all depends on what fish you want.
Seriously, alot of fish can live outside their parameters as long as they remain STABLE. Which means ensuring KH
isn't too low.
My advice - buy a water test kit at your LFS
, it will tell you what your water is so you can choose your fish appropriately (if you wish) and will give you hardness numbers that are acceptable and unacceptable.
(I'd give you numbers but I am in Canada and the units would be different making the numbers useless to you).
20 gallon FW
: 4 male Fancy Guppies, 2 male Platys, 6 Neon Tetras, 1 female Dwarf Red Gourami, 1 male Dwarf Powder Blue Gourami, 1 Bristlenose Pleco