We generally don't like to get into this type of thing as its completely different and usually causes arguments between forum members but to answer your question in a round about type of way all i can say is that your GPU is undoubtedly the better hardware as far as actual rendering capability and flexibility is concerned.
I will balance that by saying that the optimisation of the 360 games and the 360 graphics processing hardware means you get very decent graphics which are fluid and look good. It should as the whole package is basically (other features aside)optimised to do one thing which is run 360 games.
So depending on the other hardware in a PC which isnt optimised and is more of a jack of all trades and master of none you could very easily end up with a game that doesnt look as good (this is all down to personal preference as well) or run as well as the same title on a 360.
There really is too much difference for a straight comparison.