Plasma's do better with contrast, much like a CRT, than LCD's do. However, high end LCD's have high contrast ratios. So if you end up with a LCD with 10,000:1 you're in good shape! Even if it's 7,000:1 or more... But 3,000:1 and you'll wish you didn't buy it.
Plasmas have a shorter life than LCD's. Plasma's also have screen burn, where LCD's don't. If you are using it for TV or movies only, then the plasma might work well for you. But if you intend on playing video games, then you need the LCD. However, LCD's are usually slower and can have ghosting or image problems because it can't draw/refresh fast enough. But again, high end and you won't have the problem. 8ms or less is a good fast speed for a LCD....
Also look at the screen resolutions... Some 40" TV's ony have 768 pixels, while the 32" do as well. Meaning, more pixelation to achieve the larger screen. 1080 is more appropriate for a 40" TV.
So... If you find a LCD with 10,000:1 contrast ratio and 8ms or faster screen, then you will have a nice LCD TV! Check out the 96 series Samsung LCD's.....A 40" is about $2,000.
Here is my reccomendation....
http://www.crutchfield.com/S-MeVfaTcNHt4/cgi-bin/ProdView.asp?g=146350&I=305LNS4096
Free shipping and in home setup as well....