Calculating differences between documents isn't something XSLT itself is designed to do. Folks have attemped to write tools which will help you generate stylesheets "by example" -- I believe one of these appeared on IBM's alphaWorks website some time ago (http://www.alphaworks.ibm.com) -- but that doesn't seem to be what you're looking for; you say you want to calculate numeric differences between fields in the two documents.
I think your best approach is probably going to be to write a dedicated applications, probably based on an XML parser such as Xerces and some specific DOM or SAX code.
