Robert Sheldon has an article on collations:
The ideal solution is to choose a collation when setting up SQL Server that can be used for all your user databases and character columns. Using one collation removes any issues you might encounter when querying the data in different ways. It can also be the best approach in terms of performance if multiple collations impact your queries. However, this approach works only if the same language and collation settings are appropriate for all your users and applications—or at least a good majority of them.
If you support multi-cultural environments, you’ll need to take into account a number of considerations. To begin with, you should pick collations that support the most users, and you should use Unicode data types where possible because they can help avoid code page conversion issues. Just keep in mind the storage requirements that come with Unicode’s two bytes per character.
My inclination is to say Unicode everywhere possible. There are cases in which Unicode doesn’t fit, but it’s easy to do and if you have enough data to worry about the extra bytes Unicode characters take up, Unicode compression is available.