To put this simply these are both very good lenses the difference is




That the 24-70L. Is a pro series lens its fully weather sealed. Made Metal and quite heavy I would say it also slightly out proforms the 17-55 optically. On the other hand the 17-55 is an efs lens so it not compatable with and film or pro series cameras.
Also It is a good quailiy lens but its plastic not weather sealed and is made more for consumer used than professional use.




A good note for the most part L series lenses are, but not always better at taking pictures but it will always hold up beter in the long run. One of my 24-70 2.8 is now five years old and my 28-135 is which I bought before my 24-70 broke within 2 years of professional use the build quality is just better.




I hope this explains the difference






Thanks.
Coastal Kid