I am in the same boat - looking for either a 50" or 57" and debating which brand. I am no expert, but have been researching for a couple of months.
The main differences on the Hitachi are the cabinet and a built in digital receiver. The following link from Hitachi covers the differences.
www.hitachi.com/tv/browse/projection/16-9digital/16-9digital_index.shtml
The upper end sony (I forgot the model#) that has the memory stick input is not available in the 50" model - only the 57" and 65".
I am currently leaning toward the Hitachi. Cnet and Consumer Reports rated it very high. The Sony scares me a bit due to the flicker issue - but I am a big sony fan.
Let me know what you do and how it turns out. We will probably make our purchase sometime in June (unless we get a Sony and wait for the new models to come out in July).
Matthew