I faced a similar choice a few weeks ago and eventually decided on a new Beovision 8-40.
I like the styling on the BV8 as it reminds me (not sure why) of a 'normal' TV, not a generic panel. It's also the cheapest in the range and that made sense for me as I'm not a massive TV viewer or into the home cinema experience. In essence I saw it as the nearest replacement for my defunct MX7000, albeit with a bigger/modern screen.
I'm very happy with the choice; the picture is excellent even in standard definition and the sound is quite good too. On the downside, I'm not too sure about the rubbery texture of the case material as it attracts dust and has a sort nap to it which makes it look like its marked if you touch or clean it.
Peter's owner's report of the BV-10 tells you all the details you need to know. I believe he concludes that it's a superb picture but has limited sound, mainly due to the physical limitations of the speaker system. I believe that the chassis is the same as the BV-8, but of course it uses a backlit LED panel. It costs £2500 more than the BV-8, so it's down to personal opinion as whether that's worth paying for differences in screen and styling.
The BV-7 has a more advanced internal processing system (Beosystem 3?) which provides a full home cinema platform. A new one didn't make any sense in my personal circumstances at £10k. A second hand one probably costs the same as a BV-10 and at that price I felt the BV-10 made more sense - more up to date screen technology, great design and of course it's new with longer warranty etc. A new BV-7 is due out which may push down the price of the second hand ones making them more attractive.....
AP