They utilize the same screen and I really don't think one is much better than another . Perhaps they are running on different settings and modes.
By nature , even within the same TV models and same settings there are going to be some unit variance .
One may be a little bit more colorful than another . But the difference is not going to be huge .

The unit you will receive from the shop is going to look a little different as well unless they are sending the demonstrating unit to your home.
At this price range , just get whichever model you can find that is cheaper .

Guess if u're talking bout picture settings.. u cn check out Rtings tv settings for the ru7100 as a guide but do take note each TV will hv a diff calibration tweaks.  u cn play around with the settings to get the best optimal picture quality.  👍

https://www.rtings.com/tv/reviews/samsung/ru7100/settings

Default settings is good for starters but if you want closer to actual color & best image then you have to play around with the expert pic settings.  

And yes the TV speakers are good when cranked up about 40-60 volume.

Samsung one remote is clunky as you've already known now so 3rd party remove / air mouse works well..

TV size depends on how far your sofa is located from the TV. Modern 4K tv's requires to sit close to enjoy the best pic quality.