The pre-configured update sites are the once that match best for the ivy 5.1 designer as they rely on the correct platform (eclipse Juno). So basically it is a good idea to use them.
Anyway the error occurs because one of the sites seems to have a broken repository index. To walk around this problem you should disable the option "Contact all update sites during install to find required software" on the first page of the update wizard. Then the installation of M2E, or an update of Subclipse (SVN) works for me as expected.
![disabled dependency resolution on other sites][1]
[1]: http://answers.axonivy.com/upfiles/updateSizeWizard_deactivateDependencyResolutionInAllSeites.png
