I will answer a few of your direct questions before I get to 3D:
-The UN55D6420 is just Best Buy's version of the D6400. No differences.
-The LG LW5700 is a LESSER tv than the LW5600. The 5600 is the common one, the 5700 is for clubs/warehouses and has a lower contrast.
Now we're talking 3D. There is so many different ways to do 3D, and each technology has a lot of pros and cons. There are different types of screens, glasses, lighting, and even ways of fitting both the left eye image and right eye image onto the screen at once.
Rather than me try to explain everything to you I'll give you two good links. http://news.cnet.com/8301-17938_105-104 ... 3d-tv-faq/http://www.best-3dtvs.com/guides/3d-format-guide/http://news.cnet.com/8301-17938_105-200 ... =mncol;txthttp://www.youtube.com/watch?v=B9kHiJ2kvDQ
I can answer any additional questions you have, but 3D is still a new and unsettled technology so it's hard to explain all of them at one shot.
The move all the manufacturers want to make is towards no glasses, but there is still no good way to send the correct image to the correct eye, especially when viewers are sitting at different spots in the room.
The major difference in 3D sets is what kind of glasses they use - active shutter lcd glasses or passive polarized glasses (like the movie theater). Both have pros and cons. Active was out first and has received the most development. Passive really only has two companies making it now - Vizio was first and LG has really picked up the torch and ran with it this year.
Currently, all the premium tv sets use active shutter glasses. Depending on the source, it allows each eye to see 1080p lines or resolution. The LG set you mention has a film on the screen that polarizes each line of pixels, essentially sending each eye 540p. Allegedly with a new firmware update you can get 1080p to each eye by turning off their TruMotion effect, but I have no got to play with it enough. Beyond that alot of the 3D content (video games, 3DTV) is not in true high definition. ESPN 3D will only send you 360p lines of resolution to each eye. Sports are really cool in 3D, but it is a much more subjective experience.
The Samsung glasses now sell for about $30 each and require a watch battery. The LG ones are the same sort you get at the movie theater and are only a few dollars each. Passive has less crosstalk between the two images and is a bit brighter, but this comes at the cost of contrast and HD quality.
I've only had access to an LG Cinema 3D set for a few weeks so I am not the best judge of it's quality, especially in 2D. We've had their demo loop or Avatar on it in 3D nonstop. The D6400 however has an excellent 2D picture. CNet rated it's 3D picture better than the W5600 and also Panasonic's entry 3D plasma the ST30.