Denton is 100% right. AUS 6 I wouldn't bother with, AUS 8 is OK, but 440c is the best among them, and likely the best stainless steel you'll find in a factory-made blank. It will usually only be a few bucks more for a 440c blade, and worth every penny. It will take a finer edge and keep it much longer. It will be just as corrosion resistant, probably even more so. There are still custom makers who use 440c, it's not the newest "super steel" but it's good stuff.
If the ad only says it's "stainless" but doesn't specify the grade, it's junk! Any maker or manu who spends the extra money to use quality steel will be proud to tell you exactly what it is. Even if it just says 440, it's junk. 440c is the high-carbon version, which makes very good blades... if they call it 440(no letter) it's probably 440a or 440b which are not nearly as good for knives.
The reason factories often use lower grades of stainless is that it's cheaper to begin with, and significantly easier to stamp/grind etc. When you're making 1000's of blades at a time, this saves barrels of cash in terms of time and wear-tear on the machinery. Unfortunately this results in a blade that doesn't hold an edge nearly as well as it would if it was made from high-grade, high-carbon stainless.