Besides local dimming here are some more differences between the TCL 5 series and the TCL 6 series. I did this comparison using the Roku versions of these TV's with 75" screens (75S555 and 75S655) but I imagine they hold for the Google versions as well. Of course weight, Vesa mount, and power consumption will be different for other sized screens.
Display
75S555 QLED, up to 40 contrast zones (note: last year's 5-series model had 80 local dimming contrast zones); 60 Hz refresh rate
75S655 mini LED, up to 360 contrast zones; up to 144 Hz refresh rate
Remote Control
75S555 RC280 (no voice)
75S655 RC580 (voice control)
Stand
75S555 "chicken leg" stand, two mounting positions: wide spaced and closely spaced
75S655 center pedestal, adjustable height
HDMI ports
75S555 4 HDMI ports at 60Hz, 1 of which is eArc
75S655 4 HDMI ports, 2 at 60Hz, 2 @ 144Hz; 1 of which is eArc, and the eArc port is 60Hz, could be a problem for people that use their AVR as the connection hub.
5-series weight with stand = 66.4 lbs.; weight without stand = 65.3 lbs.
6-series weight with stand = 77.4 lbs.; weight without stand = 70.3 lbs.
(note: weight is for 75" TV's)
5-series has 400X300 Vesa mount
6-series has 400X400 Vesa mount
(for 75" TV's)
Power consumption--75" panel
75S555 196 watts (per energy star guide ratings)
75S655 341 watts (per energy star guide ratings)