Best answer is, it can make BIG difference, but not always.
Some of the average stuff found at supermarkets isn't bad, but the problem is, some of it is nothing but horrible.
A couple recent examples...I helped a friend get started in sausage making, among the ingredients he'd purchased locally was some whole nutmeg, everyone is told that it's best to buy whole nutmeg and grate it fresh and I agree....but the Badia nutmegs that he purchased were very bad...small, pale, musty tasting and largely flavorless......same deal with the ground sage he purchased....almost no flavor.
You won't end up with the best final product when using junk. It's impossible!
Garbage In=Garbage Out!!!
Unfortunately, there's no way to know what's good or bad unless you buy it and try it, I've found that to be more expensive in the long run than just buying good stuff to begin with, and it's a heck of a lot less frustrating!
As far as the Kosher salt goes...Morton and Diamond Crystal are the most popular.
They're both fine.
I keep those on hand as well as David's because they're all a little different texture-wise.
Here's the link to Penzey's Spices....
http://www.penzeys.com/
~Martin