Main difference is, hard you tend to use less, easier to get a thinner coat and tend to be a little more durable. Also more suited to using a foam applicator.
Soft waxes are more suited to bare hand application. Tend to clog up a foam applicator if you have a tendancy to keep going back to the pot. You'll be supprised how easy this is to do!
Other then that, the main thing for you to decide is your preference.