I've spent some time comparing the 2 models.
As to which one to buy, it's a coin toss.
If you spend enough time researching and viewing any current HDTV, you'll find that the perfect TV does not exist today. Each technology, DLP, LcOS, LDC, etc. has it's strengths and weaknesses. The same with each manufacturer, Sony, Mitsubishi, Samsung, etc. has their good points & weaknesses.
I'm going to be buying in a week or two and here's how these 2 models stack up.
Features such as HD tuners, connectivity (2 HDMI, component in, etc. seem similar to me). Brightness & contrast also seem more than adequate to me on both models.
Larger screen (52" vs 50") in the same size box (within 1/2" in HxWxD).
A lot of TV for the $$, local street prices here is $400 less than the KDS-50A2000
Has the same "wavey-wateryness" that I see on all DLP TVs on some scenes.
DLP light engine has the color wheel, which adds moving mechanical parts that can fail.
Some people see color rainbows on DLP - I don't see any on the 52631, but you might.
Mitusbishi has taken some knocks on reliability
Sony reputation for quaility & reliability
3 panel LcOS system has less moving parts, should be more reliable.
No color rainbows with LcOS display.
Smaller screen 50" vs 52"
Costs ~$400 more
Potential "Green Blob" problem that many SXRDs have.
An important capability of any HDTV is the ability of the computer(s) inside them to scale the input signal (SD, 720pHD, 1080iHD & 1080pHD) to the native display (1080p). From my viewing both sets show 720p HD very well and do a good job on SD. The Sony looks fantastic on a real 1080p source (Sony demo disk). Unfortunately I have not yet seen the 52631 running a true 1080p source.