Hope you know the C models are last year's and the D's are this year's. Since no one's giving you any help, let me say that most D owners on the forum say they're better than similar C's they had last year.
I believe your C model had something called precision dimming which is a firmware attempt to make blacks blacker. A D6500 doesn't have that feature, called microdimming this year in the D7000 line. Unless you're a videophile, the D6500 will probably look great.
I don't know anything about the LG. Generally speaking, Sony and Samsung are considered top tier brands by pro reviewers. LG is considered a solid second. I've helped set up two LGs for friends and they have a very good picture. Both had matte screens, however, and I like the "pop" the Samsung glossy screen produces. Go to CNET and search the web for reviews of the LG.
I'll be asked to recommend a set this year for a close friend and will probably recommend something in the D6000 line... more reasonably priced than upscale models. Good luck.