Plasma. Better blacks and contrast ratios at this point in time.
I purchased (and replaced after theft) a 42" Panasonic plasma not too long ago. It is by far the best television I have ever had.
Image retention is not very noticeable at all and there is virtually zero chance of burn-in anymore since just about every plasma manufacturer uses some sort of method to resist it (pixel shift, etc.).
The plasma picture looks more "alive" and more "real" to me than the LCD I have in the other room. It is VERY nice.
If you're buying that big though and dropping that amount of cash make sure you get one with 1080p capability beause Blu-Ray is not far off from becoming the new standard to the point of being so affordable everyone will have them and you might regret not being able to take advantage.