What am I looking for?

[openingshok]

I rented a 70-200 4.0L and took the same shots with the same settings in multiple lighting environments with it and my kit 18-135mm. All shots look identical when placed side by side. So, what am I looking for. Why should I pay 2x plus for a lens that produces the same quality as my kit lens? And I'm sure there will be some that come on and say "well, it's a rental. I'm sure it's abused and flawed". I am not posting this because I want to snub the high dollar equipement out there. I am posting this because I truly want to provide the best quality product to my clients and be able to offer them the best value for their dollar. If I buy L glass I will have to pass that expense on to my clients. but if I can get the same result from non L, and even in this case KIT glass, I don't see it as a justifiable investment. This is the first time I have shot an L, and by all the threads on here and other forums I was expecting MUCH more than what it offers. I am beginning to think this is the same thing as Buick Vs. Oldsmobile. More bells and whistles, but essentially the same car with a higher price tag. I don't get it.