Recommended approach to monkey patching a class in ruby

I've noticed that there are two common ways to monkey patch a class in ruby:

Define the new members on the class like so:

class Array
   def new_method
     #do stuff
   end
end

And calling class_eval on the class object:

Array.class_eval do
   def new_method
      #do stuff
   end
end

I'm wondering if there is any difference between the two and whether there are advantages to using one approach over the other?