How to know what light soy sauce had higher quality?
Offhand, I would guess the Pearl River Bridge is a higher quality - it has more soybean, does not have sugar, does not have flavor enhancers.  The fact it doesn't have distracting flavors makes it harder to cover up lower quality - so they must be pretty sure of their product.  

I would think higher soy percent is better simply because it's more concentrated, you get more flavor per use and more flavor and less water per bottle - you can easily dilute a more concentrated sauce, much harder to concentrate a dilute one.

The Lee Kum Kee looks like it's more dilute, and uses extra salt, sugar and flavor enhancers to bolster the taste - instead of tasting like, well, soy sauce.  These sorts of things make the product taste good, but that means they can make a "good enough" product with lower quality ingredients, usually that means they do not have to depend on the quality of their soy sauce to stand alone.

You may actually prefer either, depending on your palate.  Some people like the plainer taste by itself and would prefer the less doctored (and higher quality) product, some would like the flavor profile of the more tweaked version and are more indifferent to quality variations.  But, that is what I would see when looking at those labels.