I rate the LG G Pad 8.3 a bit on the low end of the quality scale when compared to the top tablets like the Nexus 7 2013 and iPad Mini with Retina. I haven't see the Galaxy 8.4 Pro, but I hear nothing but great things about it's display.
The G Pad 8.3's problem for me is it's black levels. The blacks are crushed and it's hard to impossible to make out details in dark scenery. It also has problems with it's white levels too from my testing at
LCD monitor test images.
Fortunately the issue isn't a problem with most of my usage. Web browsing, watching YouTube, reading books, comics, etc isn't really affected by the poor black levels normally. So the display looks good enough. But it definitely trails behind the quality of the Nexus 7 2013 and the more expensive iPad Mini with Retina and apparently Galaxy 8.4 Pro too.