Best to take a look in the review section, owners threads and manufacturers sites.
Panasonics have deeper blacks but Samsungs can go brighter, both have good contrast though having deeper blacks can add depth. Looking at the review results the Panasonics do better out the box for colour, though some calibrators seem to prefer the Samsung for calibration results. The Samsungs were a lot cheaper but theres some great deals to be had on Panasonics now. SD on Samsungs is meant to be slightly better, though I cant see it being by mutch, depending on the content SD on my GT can be ok to very good. The differances betwean the two makes isnt huge, this is why you need to demo ideally as it largely comes down to what you prefer. Styling wise many prefer the Samsungs dues to all the flash trim etc, I find it a bit naff myself and prefer the more suttle styling of the Panasonics, both make very thin plasmas, as thin as may LCDs.
Issues are well known with the Panasonics, most are fixed or have been improved to a point where(to my eyes atleast) they aren't noticeable.
Samsungs are having issues too, basically all sets of all types have issues of some sort.
On 3D the Panasonics have the lead in most areas though some find the Panasonics a bit too dark, guess it comes down to environment to a degree, the Samsung can go brighter though.
For your uses Id definitely go plasma, motion is better for sport and lag is lower for gaming.
D6900 vs D8000 Im not sure theres a great differance, best asking on the owners threads. ST vs GT The GT has a better filter, slightly better colour repro with mor picture controls, THX pre-set anf two extra pro modes. The GT30 also has freesat as well as freeview and other features I cant remember off hand. The ST has lower lag for gaming but lag still low on the GT with skightly better PQ. Features wise like I say, best to do a compare on the Panasonic UK site, see what you do/dont need.