Hi Gebbel,
Scart & co-ax are analog connections, so the signal from the Sky box needs to be converted from digital to analog, squirted down the scart / coax and then converted back to digital to be displayed on the Plasma. These 2 conversions can lead to degradation of the signal.
HDMI is digital all the way, so the "native" digital picture is sent directly to the TV with no conversion required. This (in theory) should eliminate loss due to conversion and give a better SD picture.
I have SKY HD on a Phillips 50" full hd (1080p) plasma and the difference between SD on the standard box and SD on the HD box is noticeable but not dramatically so.
HD channels look absolutely superb, but with the size of the TV, any defects in SD picture quality will be more noticeable.