Hmm... Well if by different you mean 99% of 10 year old 75's are rust free with perfect sills vs. high numbers of X-types with rotted out, MOT failure sills, yes I agree they are different quality
I suppose if you are buying a young one without the intention of keeping it too long, it might not matter to some...
Anyone considering an X-type bucket really should google: "jaguar x type sill rust" and click on images, I hope you have a strong constitution though! The X-type really does have very poor corrosion resistance indeed...