[Galaxy Tab S Q&A, Help & Troubleshooting] ukael:
I can't compare with the tablet Z, but I own a Tab S 8.4, a Nexus 7 2013 and a Note 8.0. I will try to answer your questions based on my experience:

The tab S 8.4's screen is a huge disappointment to me. It looks awesome in bright environments, the high pitch and vivid colors are pleasant to my eyes. However, for someone with high expectations as I did, it suffers from several issues that used affect previous AMOLED generations and totally ruin my experience:

- Smearing on black backgrounds. An issue that was already reported on Galaxy S4, and that seem to affect Tab S 8.4 as well: A dark red smear is visible when watching movies on black backgrounds. It looks like black pixels are slower at displaying a color than already lit pixels. The result is not obvious on first watch, but once noticed becomes a nightmare in dark movies.

Here is a fix for this issue: http://xdaforums.com/showthread.php?t=2259775

While this seem to be pretty efficient fixing the smearing issue, the result is not perfect, it is rather annoying to use AND we lose the benefits of perfect blacks in the darkness. And it impacts battrery life, as a dark grey pixel is powered, while a black one is not.

- Mura effect. Basically, dark (but not black) colors are not homogenous on the screen. It is only really noticeable in the darkness, wich is my main usage for the tablet. Rather annoying, especially since the fix for the smearing issues induces replacing all "black" by "extremely dark grey", which reveal this issue.

More information on mura effect: http://xdaforums.com/showthread.php?t=2108073

- Yellow/greyish tint. This one is'nt the worst, but can be felt on white backgrounds. This one becomes obvious when compared directly to a more natural white screen: the middle of the screen seems more yellowish and darker than the borders. It seems like the glass could be dirty, or tinted by years of heavy usage, which it is not.

None of these issues are obvious to someone who doesn't set his standards very high. They are for me though, and will probably be noticed by people willing to use the tablet in the darkness.

As a bottomline, I actually prefer the display offered by my Nexus 7 or iPad mini retina rather than the Tab S 8.4's. They lack the "wow" effect of vivid colors and the perfect blacks, but provide a more homogenous experience, especially in the darkness.

Now for the performances, I don't know what's wrong with this tab. I was thinking about putting the blame on TouchWiz, but alternative launchers don't help. Rooting and turning off as many services as possible didn't either. Here is what I noticed:

- System zoom effects are subjects to stuttering (opening new app/window, for example). None of my other android device suffers from similar issues, and zooming in other apps such as web browser works flawlessly. The issue is not reliable, and seems to reduce when the same window is opened and closed several times in a row. Memory access issue causing stuttering ?

- Overall system doesn't feel as smooth as it should. Even my elder Note 8.0 feels butter smooth compared to the Tab S 8.4, which I can hardly explain. It is fast enough, but the FPS seems to be lower.

- Two of my favourite apps suffer from low FPS when scrolling: Perfect Viewer and Moon+ Reader. The poor performances make the experience with these apps less enjoyable than on N7 or Note 8, which his hard to explain considering how much more powerful the Tab S 8.4 should be.

- 3D gaming is not at its best either. Performances are clearly below N7, and maybe even lower than on Note 8. There, I guess this is the cost for the extreme pixel density of the display panel.

If you are satisfied with the services offered by your Sony tablet, I'd suggest to stick with it. The Tab S 8.4 is a very decent tablet but not perfect, and its major strenghts are also its main weaknesses.