levival,

I can compare the Samsung specs if you'd prefer. Both sets would actually be very good for your situation. Plasma is a better "dark environment" television, in my opinion.

Of course, the screen size on the first model is larger, and that will depend on how far back you're sitting. For some people, "bigger is better", however I might suggest going to look at a 58" TV in a local retailer, stand back where people will be sitting, and see if you're close enough to have to look back and forth across the screen. If you have to constantly move your eyes, long term viewing may cause screen fatigue. So that extra 8" may be impressive, but may also take away from the movie-watching experience.

The B850 will add the Ultra Slim design, and raise the B550's 2M:1 Dynamic Contrast ratio to 3M:1. The Wide Color Enhancer will also be improved, which - if you're particular about calibrations, may offer you more picture options.

As well, the B850 will give you DLNA, the Content Library and USB functionality. The B850 will also give you Internet@TV widgets. If this is something that interests you, these are the available widgets for the 2009 models:
http://www.samsung.com/us/internetTV/

Does that help? We appreciate you considering Samsung products.

--HDTech