The JS8500 was an edge lit model, TVs like the Q70R have full array local dimming which is a big improvement.
If you find on a TV that is edge lit the backlight control is good, you'll only find its better with a TV with direct lighting and good local dimming.
Where you may be more concern is Samsung's recent trend of fitting their less than 55" models with only 60hz panels instead of 120hz like your current TV. Limited to 49" the Sony XF9005 or new XG9005 are probably going to be better buys because of this...unless you are not fussed having a 60hz panel.
As good though as LCD TVs can be, I myself own a TV which is direct lit, with FALD and 512 zones and you are still limited to dim and brighten each zone independently so there will never be perfection between one line and the next on these TVs without being able to notice where one zone starts and another ends with some content..that doesn't change even with the very top end of LCD TVs though.