As far as actual picture quality goes, they're very similar.
The F6400 and F7100 both share the same Micro Dimming capabilities. It's not until you look at the 7500 (Micro Dimming Pro) and the bread and butter 8000 (Micro Dimming Ultimate + Precision Black) that you start to notice a difference. The Micro Dimming on the F6400 and F7100 both adjust the brightness and contrast in 300 zones. The 7500 adjusts for not only the brightness and contrast but the color and sharpness as well. In addition to that, it does it in 600 zones vs the 300 on the F6400/F7100. The 8000 is the same as the F7500, but also features Precision Black which enables it to actually turn certain LEDs off entirely, but this is generally only used to to completely black out the letterboxes on the top and bottom (Cinema Black is the feature it's called in the menu).
The F7100 is a 240-hertz panel compared to the F6400 which is a 120Hz. Obviously this doesn't necessarily affect picture quality to most people, but if you're using the TV for gaming or a ton of sports I highly suggest the 240Hz.
Lastly, the F7100 features Samsungs' Ultra Clear Panel which is different than the matte panels found on the F6400 and below. The panel itself actually absorbs light so it reduces reflections quite a bit. They claim that it also produces better black levels and contrast but personally I haven't seen the two TVs side by side to really be able to distinguish a difference and form an opinion.