LA46D550 is CCFL backlit LCD with a matte screen, 60Hz, cosmetically inferior with a thicker bezel. Media playback via USB and DLNA, with the option of wireless. 4 x HDMI ports.
UA46EH5000 is LED edge lit LCD with a glossy screen, 60Hz and 100 CMR advertised as improving motion, cosmetically superior with thinner bezel and newer design. Media playback via USB, 2 x HDMI ports.
The UA46EH5000 seems to be less deep in dimension than the LA46D550 (taller & wider) which is strange. On the plus side this might help improve LCD light bleed and LED light distribution when viewing blacks in dimmer conditions, something many previous LED-LCDs suffered from and still do.
http://news.cnet.com/8301-17938_105-20088479-1/is-lcd-and-led-lcd-hdtv-uniformity-a-problem/
It's recommended you test the TVs of your choice in a non bright store, but rather dim environment/test room before making your purchase, assuming you will at times and perhaps most often use the TV at night in low ambient light conditions. You also need to take it out of Store mode/Dynamic contrast mode which all TVs are set to on the showroom floor to stand out to buyers. Store mode AKA Torch mode pumps contrast and backlight brightness resulting in overly bright contrasty colours with all finer details washed out, far from picture quality. By all means if you will be using the TV under bright conditions and mainly in the day, you wouldn't need to worry about testing in a dim environment and inconsistent blacks.