HI
The C10 and X10 you are talking about must be the 2009 model plasmas. Even so, the price you are getting is really good. But as Bublu Bhuyan will tell you Panasonic plasmas of 2009 allegedly suffer from black level problems. I don't know if this is really true but it's your call.... though I doubt you will notice any fading black levels unless you happen to be an expert TV reviewer who can make out such fine details.
In 42" screen size there will be a marginal difference between HD and HD ready. Whether you will be able to make out the difference depends on how trained your eye is and how close you are sitting to the screen.
I think the reason no one can make out the difference between s10 and x10 is that the source is at 1080i. So the Full HD is not being used to its full potential. Ask the dealer to connect a BluRay player and see if you can spot any difference. Again depends on you. For normal aam jaanta aadmi there should hardly be any discernible difference.
Full HD LCD as compared to Plasma is again a debatable issue. There will be an equal number of members ready to brandish talwaars in the defence of either technology, so it is better to go to the store and see which one you like better. But if you are ready to spend so much money on a 40" LCD then you can consider Panasonic's V20 plasma. According to the reports of forum members they are really good