well if budget is not a issues and at that viewing distance u can go for LCDs above 46inch or a 50inch plasma,at that distance of 20feet u wont be able to differentiate b/w HD and FHD nor will SD be different in both HD and FHD tvs at that distance,but by chance if someone sits close to around 10feet or less FHD makes sense.
now as far as LCD and plasma goes some plasmas like pioneer kuro still have the edge,while normal HD plasmas like panasonic PV8/80 have a contrast less then todays lcds from samsung/sony.while the FHD panasonic plasma has a 10% edge in contrast compared to conventianal backlight lcds from samsung/sony.
BTW sony has launched the new W450A series with 100hz motionflow and the new Z series with 200hz motionflow,sadly the Z series is more expensive then the X series.