Burn-in is not an issue anymore with newer plasmas, & certainly not with the Pioneer. Yes, LCDs use less energy, but that translates to savings of about £10 per year!!
You can check the TVs at local Sony stores for bigger screens. Next time you go to Currys, compare and LCD TV with a plasma (not the cheapest ones, but ones which are reasonably priced) side by side. When I was buying my TV, I did the same, & found the colours on plasma TVs to be more natural. Plasma TVs inherently resist motion judder compared to LCDs, apart from giving deepest blacks and more natural rendition of colours.
You may be able to see a Pioneer TV at Richer Sounds or John Lewis.
If 46 inch TV is good, 50" will definitely be good. It's only a 4" difference in screen size.
Uusally, if an older model is replaced or its' production being stopped, the price of the TV goes down. But in case of Pioneer, the prices have actually gone up, because everyone knows that Pioneer TVs are the very best in the market and are yet to be surpassed. Google Pioneer Kuro TV reviews & check all the reviews. They've been unanimously voted as the best TVs ever to be made....for a good reason. It wouldn't be the best, if the screen had a burn-in issue, would it?
If you can afford the Pioneer LX5090, go for it. If you're willing to consider plasma TVs, Panasonic and LG are worthy choices (Panasonic is the next best after Pioneer). If you're still decided on LCD TVs, Sony & Samsung are worthy of recommendation.
My Home Cinema: http://community.whathifi.com/forums/t/334126.aspx
Pioneer KRP 500A, Yamaha RX-V1900, MA R225HD front & centre, MA R90HD rears, MA AW-12 sub, Panasonic DMP-BD60, PS3, Buttkicker, Logitech Harmony One, Logitech PS3 adapter