Formtastic and simple_form are great gems, but you don't need them to bootstrap your forms. However, formtastic-bootstrap does make it more convenient to apply bootstrap.
So if you want a quick solution AND a great formbuilder at the same time you could definitively use the gem. If you want to learn bootstrap on the other hand I would recommend to get down and dirty and manually add bootstrap to your forms based on the examples given on getbootstrap.com.