Optically, not a great lot in it
Build quality, while the 35mm is good the 28mm F1.8 is excellent - it has ring type USM autofocus which is quieter and more accurate too.
It it worth the extra, only you can truly decide. Personally to me it isn't - I had the 28mm F1.8 but find the field of view provided by the 35mm more usable. It is harder to make wide, fast lenses which will be some of the cost.