There was more of a difference in Rails 2.x, since named_scopes did not execute your queries (so you could chain them), whereas class methods generally did execute the queries (so you could not chain them), unless you manually wrapped your query in a scoped(...) call.
In Rails 3, everything returns an ActiveRecord::Relation until you need the actual results, so scopes can be chained against class methods and vice versa (as long as the class methods return ActiveRecord::Relation objects, not some other object type (like a count)).
Generally, I use scope entries for simple one-liners to filter down my result set. However, if I’m doing anything complicated in a “scope” which may require detailed logic, lambdas, multiple lines, etc., I prefer to use a class method. And as you caught, if I need to return counts or anything like that, I use a class method.