Hi Ali -- fair point. In this instance, I am interested only about lens performance between the two. I have read their respective reviews on photozone.de as well as looked at them on the DXOMARK site. Seem to have good performance for a zoom, but not as steller as say the 17-55 is viewed on Canon APS-C sensors.
I really doubt you'll see much of a visible difference between the two.
I have never been a pixel peeper and am a strong believer in the content of a photograph than, say "the corner sharpness" etc.
Rather than buying all the lenses for your new camera in one go, perhaps just start with a 50mm and go from there.
You say you 'think' a zoom would be more useful for casual shots, but you might find that you are perfectly happy with a prime lens and could save yourself Â£1000. I am perfectly happy with just a 50mm and I only have my other lenses for work.
Something to think about before you get the dreaded 'gear acquisition syndrome'.