Difference between pry and pry-rails
Comparing their implementation doesn't make sense, because they're two different things: an IRb alternative and a plugin to make the former the Rails console.
The pry-rails gem simply adds an initializer to change the Rails console to Pry and adds some Pry commands specific to Rails. Pry-rails depends on Pry, and there's no reason you couldn't use Pry with Rails without pry-rails, it just makes it easier.