its hard to decide, i was having the same trouble as you and ended going with a plasma.
the lcd's, as said by the other users, have a much better picture quality. but the bad thing is that since its an lcd screen, there will be one day where u will notice a black dot(s) in the screen and to me its just very annoying lol. but ive heard this can be fixed but needs to be sent to the manufacturer.
the plasmas, they do not have that lcd problem since its mainly made of gas. the picture quality is somewhat a bit less than that of an lcd and you can get a plasma buch bigger than a lcd for similar prices. if you have a close eye on the plasma and lcd.. u will notice the plasma picture is a bit blury. one other problem of plasmas is that if the image doesnt move over X time, the image will "burn" into the screen. but alot of the new plasmas have a setting on the menu where it will "shift" the image every 20 seconds i think. but it moves it very little and its almost not noticable at all imo. they also come with other settings that try to fix the "burned" image in the screen. mine has one that turns the screen all white or turns the colors to negative every 30 mins i think.
both are great and each one has its ups and downs.
if you want to have the best picture quality, lcd is better.
if you want to have a bigger screen and not worry about black spots later on in the screen, plasma is better.