honestly this seems like a no brainer...
if the plasma was 10" smaller for 50 bux more, then i'd have to think about it, and how the tv would be used.
the way I see it
Dark room: lcd can not be used, backlight uniformity issues become too noticeable, and edgelit is unwatchable. plasma excels by comparision
comfortably bright room: lcd's flaws are mostly hidden, both displays are viable.
daylight/surgical room bright: most plasmas not bright enough, many LCD's not bright enough, no longer care about picture quality as nothing will look 'reference' under such harsh conditions.
as for motion blur, input lag, screen reflectivity, you have to assess those on a model by model basis now.
I feel like in your situation, for 50bux you get a significantly larger screen and possibly better picture too. ultimately it'll depend on what compromises you're willing to make, and most sensitive to.
Displays: Samsung PN64F8500/JVC X35
AVR: Pioneer VSX-1130K, 7.1/5.1.2 audio
Sources: HTPC(Enby), PS3, XBOX360, Wii
Control: Harmony One