What makes u say it is "a lot" better? Are u specifically referring at the edges of the lens?

Edit:
Of course, i am probably not giving enough importance to the fact that the black lines (full open) of the expensive lens are obtained at f/1.2, while they are obtained at f/1.4 for the "cheap" lens. Which should imply that at f/1.4 the expensive L lens would perform (significantly?) better. Am I making sense?

Again, thank you all. This thread is as always extremely interesting and informative :-)