My experience: Summer 2010 I had a 40" LED Samsung ($1000) and returned for a 50in Samsung 3d Plasma package ($1700: tv, 3dbluray player, 4 glasses). The reason I returned for the plasma is I wanted to do 3DTV PLAY (nvidia) on my tv, and plasma is better for 3d. But 3DTV PLAY was delayed forever, and now it kinda sucks. Only 1080p24hz or 720p60hz... 24hz is too slow and 720p looks like **** for pc games.
Plasma is great, but if you do PC desktop or PC gaming, you have to be very careful about image retention, or burn in. I actually got a slight permanent burn-in from World of Warcraft icons. The desktop can do the same thing. Also, it generates a lot of heat compared to and LED-LCD and it is much heavier. Motion on a plasma is nice and smooth but the new LED-LCDs are pretty good at motion these days, so it's a tradeoff with the other pros/cons.
I would personally recommend getting a Samsung LED-LCD like I first had. I didn't have internet apps (the new ones might now), but it did have 120hz motion which is decent, but kinda gimmicky. I loved how light it was - I actually had it sitting on my cheapo bestbuy desk like a monitor. It also was much brighter than my plasma and used much less energy in the higher brightness settings than a plasma. If you ever hook it up to a computer, it will look just like a big monitor, and better than a plasma in this respect.
I have the internet apps on my plasma and I didn't care for them, but now I actually like it a lot. I use the VuDu service sometimes, but they are pricey ($6 HDX, $5 HD, $4 SD for 24hrs) and you have to have a great internet connection for the HDX (1080p) streaming. VuDu does have a 99c movie everyday and sometimes it's good, but you can buy it and have 30days to start watching it, then 24hours to finish. VuDu also has a few 3D-HDX movies now. The Netflix is good, but not so many HD titles and overall limited on new titles. I also can do Youtube, Facebook, Googlemaps, Napster, Pandora, Huluplus, as well as some news apps, etc.
Overall, the LED-LCD tvs are super lightweight, bright, fairly smooth, and low power compared to the best plasmas. At 40" you should be able to find the price you want, and even with internet apps, just wait for them on sale at bestbuy first - and make sure they price match to bestbuy.com!