I am using POEdit to modify the localization file of some projects.
Unfortunately it seems not able to do comparisons with different versions. Do you now if it does exists a comparison tool for Windows that support PO format?
I can suggest a tool as alternative to Poedit, it's http://poeditor.com/ in which you can create more projects and compare the translation files. It also has the option of setting a reference language from other translations you have on the project, which shows the other language under your original strings, so that you can compare.
You may want to try BeyondCompare. I don't know whether it is suitable for PO files, but it may also get "trained" for such files. I know that the strings are often split into multiple lines, which may cause some problems. Otherwise, as the PO files are plain text files (in UTF-8 usually), this should be easy.
You may also want to write some perl script for this. I remember I wrote something to take a PO file and produce a printable document with the joined lines.
The Beyond Compare software is available here: http://www.scootersoftware.com/moreinfo.php
I understand that plug-ins can be written for Beyond Compare.
Have a great day :-)