If you are really going to use a plasma to watch the same normal content you would without the HTPC. In other words as a TV/Home Theater display I see no reason you would not make the same value choices you would make between plasma and LED/LCD.
If you live in menus and at the desktop and are going to be using the Plasma a significant amount of time as more PC monitor than Home Theater display than get an LED/LCD or use sensible behavior to limit image retention and burn in.The better plasmas are a tremendous value in terms of size, image quality and price.. I think the real downside beside image retention and burn in for plasma PC monitor use is their high power consumption.
I have an older Panny 58" that gets occasional use with an HTPC, but for content playback not as a PC monitor per se. It is okay. I would rather have an LED in its place because I could use it much more casually without concern for any burn in, or the heat it throws off. BTW I have taken to remoting into that HTPC using Remote Desktop with the display off when I am going to spend some significant time configuring or doing other maintenance tasks.
I asm astounded by how little power some of these 2011 LEDs use. Now I know my older 58" Panny plasma is a near worst case but my brother recently bought a very nice Panny 42" LED and it only uses about 35 watts, combined with his G620 Sandy Bridge it makes a very nice low power HDTV and HTPC combo that he uses on the wall in his office, somewhere about 65 watts total PC and display. I am using a 37" Pannly LED as my desktop display and while it gets more use as a PC, than HTPC it too is in the 70 watt category for display and i3 Sandy Bridge wth GT 545 Nvidia. I have a Denon in between the two and the Denon idles around 35 watts. Can't beat a little over a 100 watts for receiver, HDTV and HTPC.. maybe 150 watts playing blu-ray and HD audio.
My brother also has a recent 50" Panny plasma low end 1080P unit that his wife uses with an XBOX and Connect. When you start to get a lot of activity on the screen that Panny is pulling nearly 300 watts... totally different league. The screen on the low end Panny looks dirty and the image is grainy somehow. I did not try to dial it in for him, but the 42" Panny LED upstair with PC driving it blew the Plasma away for the most part. The LED was more money.
Downside of HDTVs in general as computer displays is they don't have PC type power management. I would truly like to see this implemented. Should be a setting that would properly put HDTVs to sleep when the PC goes to sleep. Yeah I know there are domre power management capabilities but they are most about powering off, not sleeping and resume is not part of the tool kit.
I have a big DLP driven exclusively now by HTPC and the downside with it, is the fact it is not instant on and off. It has relatively high power use, it uses less power than my borhter's 50" Pannythough, so it is not really a criticism based on the size. Also iwth DLP 1 to 1 pixel match means overscans. The last piece is not really a problem for me personally a I have an Iscan Duo so I can quickly scale the desktop for those tasks that need zero overscan and the scaling is so good you often forget to set it back to native..
It is an 82" and so like the saying goes about American V8s... there is no replacement for displacement I live with the not so HTPC friendly aspects on this one. Now the latest Mits LaserVue is an 85 watt device which is not bad for 75 inches. I haven't seen the new one with the clear screen (2011) which is supposed to have an amazing image. It probably is much closer to instant on and off, but the overscan caveats still apply. Using the display driver to scale the desktop to my eye is only a band-aid useful for manintenance tasks.
Happily burn in is not a concern with DLPs... Mine will get replaced with a 92" DLP later this year or early next..
I don't belive there is such a thing as perfect display for HTPC, the better LEDs come closest if they can meet your size and pricing requirements.