I went to an Apple store today to decide what to get, and none of them displayed ghosting artifacts.
I moved the about box rapidly around the screen, and it was fine. Same for mouse over various backgrounds.
Send it back.
As for reviews saying the screen lags - if it does, I can't tell, so for me it is a non-issue.
Mac Mini with 5K studio display (the 27" I think?) seemed slower than any MBP, but this may be the difference between 60 Hz and 120 Hz. Lag did NOT seem worse. In the few minutes I had, I didn't get to see if refresh could be set higher, or whether it was in low power mode.
The nano display had "different" contrast characteristics, and left everything looking slightly textured. Gloss screen looked best and had better brightness and contrast.
The difference was not insignificant; the promotion box showing dots for body color were very different.
On the gloss display, the dot for white looked slightly darker than the background, whereas on the nano display the white dot was significantly brighter and appeared almost washed-out.
Overall, the gloss screen definitely appeared to have the more accurate presentation, deeper blacks, and better contrast. Colors were noticeably more saturated.