Since there's been people saying their LG screens developed image retention after 6 months, I think there is no certain way to tell yet if all LG screens have this behavior or not. And since the image retention pattern is observed on both 15'' and 13'' from LG, it's kind of telling that the LG technology or manufacturing process for these screens is different than Samsung's.
Personally, I would stay away from components that are known to develop a problem over time... since I wouldn't want the hassle of dealing with a warranty errand that Apple may or may not honor, depending on who you speak to, whether there's an Apple store close by or not, etc, etc...