You can achieve what you want with the help of a small Python script (you'll need Python installed, as well as the lxml toolkit).

tagsort.py:

#!/usr/bin/python

import sys
from lxml import etree

filename, tag = sys.argv[1:]

doc = etree.parse(filename, etree.XMLParser(remove_blank_text=True))
root = doc.getroot()
root[:] = sorted(root, key=lambda el: el.findtext(tag))
print etree.tostring(doc, pretty_print=True)

This script sorts the first-level elements under the XML document root by the content of a second-level element, sending the result to stdout. It's called like this:

$ python tagsort.py filename tag

Once you've got that, you can use process substitution to get a diff based on its output (I've added one element and changed another in your example files to show a non-empty result):

$ diff <(python tagsort.py file1 Id) <(python tagsort.py file2 Id)
4a5
>     <AddedTag>Something</AddedTag>
17c18
<     <Role>X</Role>
---
>     <Role>S</Role>

I had a similar problem and I eventually found: http://superuser.com/questions/79920/how-can-i-diff-two-xml-files

That post suggests doing a canonical XML sort then doing a diff. The following should work for you if you are on Linux, Mac, or if you have Windows with something like Cygwin installed:

$ xmllint --c14n FileA.xml > 1.xml
$ xmllint --c14n FileB.xml > 2.xml
$ diff 1.xml 2.xml

One approach would be to first turn both XML files into Canonical XML, and compare the results using diff. For example, xmllint can be used to canonicalize XML.

$ xmllint --c14n one.xml > 1.xml
$ xmllint --c14n two.xml > 2.xml
$ diff 1.xml 2.xml

Or as a one-liner.

$ diff <(xmllint --c14n one.xml) <(xmllint --c14n two.xml)

Two approaches that I use are (a) to canonicalize both XML files and then compare their serializations, and (b) to use the XPath 2.0 deep-equal() function. Both approaches are OK for telling you whether the files are the same, but not very good at telling you where they differ.

A commercial tool that specializes in this problem is DeltaXML.

If you have things that you consider equivalent, but which aren't equivalent at the XML level - for example, elements in a different order - then you may have to be prepared to do a transformation to normalize the documents before comparison.

diffxml is one tool that does this and xmldiff another. Both of them have options for choosing what you want to consider a difference, and produce structured output. With diffxml you get an XML file describing the necessary transformations in detail, and with xmldiff either a list of XSL selectors and what to do with them or an XML file in xupdate format.

There are also libraries for PHP, Perl, Perl again, Python, C#, ..., which you could use to build something giving whatever exact behaviour you wanted.