I guess the actual culprit will be the kit lens of Canon for sure. They are optically not that good.Try them Vimal. You will be surprised.
Agree with Doc (LW). Stopped down, Canon kit lens also produces good results. One has to get used to it and identify the sweet spot. And Vimal, the quality of kit lens for Canon and Nikon donot differ. I have seen it with Riju's Nikon D90.
Arindam, there is one more issue with high resolution sensors, that is of noise. My personal experience with Canon 50D says so. I checked up with dpreview and it is suggested that for low light situations or when there is a doubt, shooting at reduced resolution yields better results. I tried this with RAW (full res.) as well as sRAW1 (reduced res. about 12MP). At poor light conditions, sRAW1 gave marginally better results. I donot take very large prints and hence this suits me. However, 5DM2 does not have this problems I heard, not tested myself though.