I just discovered File Merge (it's part of xCode, which I think you can get for free if it's not on your OS X disc.)
It will be under OS X (or your main disc name)>>Developer>>Applications>>Utilities...
Double click File Merge, drag one file to left, and one to right. If you want to see what's different in the right file from the left, then put a check mark in the Change Left box...which I gather means Changed from Left file.
Let me know if this works....it works for Html pages, and other types of files, I just haven't tried it for 2 excel files.
The dark ones are the changed ones, and if you double click, it should open another window that will have an arrow between the two files to show you where they are different.