has_many in routes.rb

Hi
   I saw
map.namespace(:admin) do |admin|
        admin.resources :accounts, :has_many => [ :credentials ]
end

and I wrote like

map.namespace(:admin) do |admin|
        admin.resources :accounts do |account|
                account.resources :credentials
        end
end

       The only difference I found is, in the first no helper methods
are generated..I would like to know is that the only difference ?If not
on which circumstances we use these two?

Thanks in advance
Sijo