Definition: class_name with a string or character in Rails 3

If I have a model Invitationand a model User, with a model Invitationhaving a column requester_id, why does this work,

class Invitation < ActiveRecord::Base
  belongs_to :requester, :class_name => "User"
end

and it doesn’t work?

class Invitation < ActiveRecord::Base
  belongs_to :requester, :class_name => :user
end

In general, when do I use characters and when do I use strings in Rails 3?