U50 is a non-3D lower cost model, so the panel doesn't have as fast response time.
UT50 is the lowest cost 1080p 3D model, but with only two HDMI ports and no wireless ethernet built-in, also no anti-glare screen filter and no custom pro settings.
ST50 is 3D and adds the anti-glare screen filter, custom pro settings, has three HDMi ports, wireless ethernet built-in.
GT50 adds a fourth HDMI port to the ST50, plus a dual-core CPU for faster internet apps, and a better styled cabinet.
The 50ST50 has gone on sale under $1000 at some stores at times, as low as $899, even the larger 55ST50 was on sale for $999 at Frys recently.
Have to keep checking for sales each week and at online dealers like amazon.
For more HDMI ports, just add a good electronic HDMI port switcher like from monoprice - I used a 4:1 HDMI switcher that also has an optical and coax digital audio output to connect to an older 5.1 receiver that doesn't have HDMI capability.