I now understand the difference between the above options thanks to SO: http://stackoverflow.com/questions/1214939/update-item-to-revision-vs-revert-to-revision
Update to revision will only update files of your workingcopy to your choosen revision. But you cannot continue to work on this revision, as SVN will complain that your workingcopy is out of date.
Revert will undo all changes in your working copy which were made after the selected revision (in your example rev. 96,97,98,99,100) Your working copy is now in modified state.
The file content of both scenarions is same, however in first case you have an unmodified working copy and you cannot commit your changes(as your workingcopy is not pointing to HEAD rev 100) in second case you have a modified working copy pointing to head and you can continue to work and commit
Hopefully this one will not catch me out again!