Wow! I just found out that BeyondCompare supports XLS files. Someone mentioned it in their forums while I was looking for an update on OS X support.
So I went and downloaded some rules so I can compare PDF files (the text only), XLS and Word docs. Yay! This is fabulous. The rules export the XLS and DOC and PDF files to text and compare the text. So what — except that I can’t edit the files directly, it’s a great time-saver!