SDI can be compared to component analoge video as you can compare Hi8 to MiniDV.
They both can carry the same resolution but Hi8 does have all the disadvatages of analoge video.
This is probably not a very good example but it does give you an idea in what direction to think.
( DV is digital video but in order to get the smallest digital stream possible it does compress
color information in a big way. You can see the result of this when trying to use a chroma key
on some DV footage
As where the quality is concerned SDI is far better than lets say RGB analoge video.
The SDI signal in a video cable is made up of 1 and 0's, this means that all the video information
is completely digitized before it enters the stream.
This includes the color coding, scaling and levels which are all digital.
It can carry a video signal for more then 200 meters whithout any loss what so ever.
This opposed to analoge component video where the cable lenght of the 3 ( RGB or Y, R-Y, B-Y ) components must be
exactely the same and no longer than 50 meters in order to prevent artyfacts in the resulting signal.
So you can conclude that both systems have the same resolution in video quality but in practice
SDI is the overall winner.
And this is not only the result of carrying the signal from one room to the other over a certain distance
It has also to do with the complete line of recorders and player mixers switchers DVE's and so on.
Because the SDI core of these systems is digital and not analoge.
I hope this will clear up some ...
I can feel it, my mind is going ....