Hi Roberto,
just my opinion - MergeTool is quite good (used it in the old days), but for an upgrade from 5.0 to 2013 you're nearly lost with about any tool. Personally, I'm using a mixture of Mercurial (Repository) and SVN (TortoiseMerge is best of breed for merges, IMO), FreeCommander, some scripts and discipline. With this combination I can get around, but it's not easy to teach to somebody else. An upgrade from 5.0 to 2013 is quite manageable there (except for reports (hopeless) and pages (no counterparts)), merge down too, but not as easy.
When it comes to translations (for other localizations), I'm using the old-fashioned export/translate/import method, but especially for this Mergetool is pretty well equipped. I haven't gotten a case to test it thoroughly, though.
with best regards
Jens