X7000e : the most basic UHD TV entry from Sony last year , second cheapest among your list , NO WCG , No Local dimming , Good motion handling , can play 24p content, great sport programs support , doesn't support WIFI 5G , the 55 inches version is a VA panel which I would recommend if you can find one cheaper .
x8000e : There have been quite some ppl singing praises about this particular model in Lowyat ( I don't get it ) to the point comparing it with OLED ( Joke of the decade )
One of them attacked me with a death threat before . It's scary when people paid a large sum of money for something mediocre yet want to feel justified for the high expenses.
Back to the TV , this is actually a decent TV , just not for its price . It used to be much more expensive. Despite being a middle level TV, the brightness of model is very meh , not to mention it has no local dimming which hampered its ability to display any decent HDR , below average contrast level even when comparing with the IPS standards . It has WCG , meh motion handling ( worse than X7000E in fact) , slightly better upscaling processor (like all Sony TVs ). Input lag is a little on the high side as well . If you can find it below 3k it isn't a bad choice , I guess
Samsung : Their lower end TV has been getting worse by each year , from the all amazing KU series to the not so impressive MU series , this year's NU series is a hit or miss
MU6100 : The best all around-TV for its value . The mu 6 series have very amazing contrast level , despite having no local dimming of any sorts . No WCG , No local dimming ( using Samsung's CE dimming ) , great BFI support
Nu7400 : Suppose to be the successor to mu 6 series with wide color gamut support , it also has steam link if you use steam . I have seen this TV from the store , it looks to be identical with 6300 with similar brightness ( which is decent )
LG : LG's Mid ~ lower end models love to use RGBW panel , buy at your own risk
UJ630 T = The cheapest TV among the bunch . This TV uses RGBW panel , which really hurts its color volume especially when showing Brighter NON-white colors , if you decide to use your TV as monitor one day , colorful fonts are going to look weird. If you can look past this , it isn't very much different than the Sony x7000e .
SJ800T = The supreme UHD LED TV from LG , of IPS panel . This is probably the best TV among your list , it has all the features required to display HDR . While it still can't really compete with higher End models, this is the most comprehensive IPS TV you can buy @ around 3k
All the models you mentioned have very good input latency for gaming except the x8000e . Sorry Sony , I still love you very much , just not your low - mid range TVs
As for screen size , Amazon recommends the distance from your 4K TV to be around 1.5x the size of TV screen . EG : for a 55 inches TV , the distance between you and your TV should be @ 55 inches - 75 inches . Although the whole point of getting UHD TV is , you can sit closer to the screen to witness the awesomeness of all the 4K pixels