I can almost guarantee that 50" 720p has a wonky resolution at 1024x768 which trust me looks like crap.(I personally don't even consider it 720p) Not to mention that the "class" plasma's are usually missing the anti-reflective coat.
Well this may just be the case with panasonics? My friend bought a Panasonic class plasma and it came with no anti-glare or anti-reflection coat and you see any and all lights on in the house through the tv.
The PN50C450B1D looks like it does come with an anti glare/reflection coat.
If your room is small enough, I'd go with the 40". A 50" will make it hard to follow, you'll sit too close, and like dude_56013 said, you could get nautious. I have that feeling too when I'm too close to the screens.