I've used and installed both models. I installed a 506xde yesterday and find it to be a most impressive screen. The biggest difference that I can see over the 505 is improved black level and contrast. I also noticed that overall, the 506xde gave a better picture than the 436xde. Also, the media-box supplied with 436 and 506xde models have 2x hdmi sockets (1x coupled with the 3rd scart - sacrifice the scart input to use hdmi) as the previous media box had only one, independent, hdmi.
I'd definitely go for the 506xde (if thats why you're asking) they are both good screens but there is certainly an improvement in the newer model.