Differences are mainly cosmetic. According to LG's website the 6400 has "super resolution plus" while the 6700 doesn't (keep in mind mfg's websites aren't always accurate) and the 6700 has Skype capability while the 6400 doesn't. Most gurus recommend turning "super resolution" off, I don't see any difference with it on or off on my 6400.
There's a half-inch border under the glass/plastic sheet covering the whole front of the 6700, while the 6400 has a half-inch black bezel and no glass/plastic sheet. The 6400 is slightly less reflective.