Rails: What's the difference between lambda, scope and class method? What's best practice?

The code snippet has three methods: lambda, scope and class method.

All of them returns the same results.

Questions:

1. Is there any best practice in Ruby/Rails when it is preferred to use one over the other ?

2. In what cases would you use lambda, scope or class method ( best practices ).

   class Cars < ActiveRecord::Base     
     attr_accessible :manufacturer, :price, :used
   
     #one
     scope :used_and_cheap_lambda, lambda { where('used = ?', true ).where('price >= ?',30000) }
   
     #two
     scope :used_and_cheap_scope, where('used = ?', true ).where('price >= ?',30000)
   
     #three
     def self.used_and_cheap_class
       where('used = ?', true ).where('price >= ?',30000)
     end
   end
   
   
   
   
   
   Cars.used_and_cheap_lambda.count
   => #24
   
   Cars.used_and_cheap_class.count
   => #24
   
   Cars.used_and_cheap_scope.count
   => #24