My understanding is probably about 6 months old which is a long time in this rapidly changing field. Caveat Emptor.
Size. In general plasma TV’s are larger and LCD’s are smaller. The break point is right around the 42” that you mention so you probably do have a reasonable choice between the two as long as the industry hasn’t drastically moved in the last 6 months.
Cost. In general the cost per size ratio is comparable between plasma and LCD, however for the same size screen you’ll probably have to pay a couple hundred more for a Plasma TV versus a LCD TV. Given your 42” for $1000 metric this may restrict you to an LCD TV.
Color. LCD requires backlighting whereas Plasma does not. This means that colors might be slightly muted and black may be merely dark gray (i.e. lower contrast) on a LCD TV versus a similar plasma TV. In all honesty I found this to be a very small difference as long as the LCD TV isn’t the cheapest one on display but it *can* be noticed.
Motion Artifacts. LCD TV’s have a tendency to have difficulty displaying certain scenes containing movement. The classic one is someone in a striped shirt swinging on a swing. This can result in apparent disconcerting jumps in the picture. To get around this the better LCD TV’s refresh at 120Hz as opposed to 60Hz and this is generally sufficient to virtually eliminate the problem. If you go LCD I highly recommend 120Hz over 60Hz. Plasma TV’s do not have this issue. I believe this is the stuttering issue that Motti mentions.
Viewing angle. A long time ago the difference here used to be extreme but LCD’s have caught up significantly to Plasma’s recently although Plasma’s do still have a slight advantage. The standard LCD viewing angle is 160 degrees although some models boast up to a 175 degree viewing angle. Plasma’s have up to a 178 degree viewing angle although it *is* possible to get an LCD with a better viewing angle than some Plasma’s.
Lifespan. Lifespan is defined as the number of viewing hours a TV provides until the overall picture dims to half brightness. This is an area where LCD has historically had the advantage over Plasma but Plasma has pretty much caught up. For LCD’s the standard metric is a 60,000 hour lifetime which is 27 years of 24/7 viewing. You can now get Plasma’s in this range but the average may be somewhat less.
Burn In. I believe that Motti correctly and sufficiently covered this.
Pixel Issues. LCD’s may have stuck (always on) or dead (never on) pixels or various combinations given that every pixel consists of three sub-pixels (RGB or CMY - Cyan, Magenta, Yellow). At reasonable resolutions dark pixels can be difficult to notice but stuck pixels are really annoying. To me this is the single biggest concern when buying an LCD. Carefully read the literature on the specific model you intended to purchase and fully understand the manufacturer’s pixel defect policy. I would never accept a TV with a stuck pixel of any kind nor more than a handful of isolated dead pixels. To the best of my knowledge Plasma’s do not have this issue.
The only other thing I can think of is weight, power and altitude. Plasma’s are heavier, more fragile and consume more power. Plasma’s may not work so well above an altitude greater 6500 feet. LCD’s are lighter, more robust and consume 30 to 40 percent less power than a Plasma of similar size and are not affected by altitude.
Happy shopping. 
