Using a surrogate key is better in my opinion as there is zero chance of it changing. Almost anything I can think of which you might use as a natural key could change (disclaimer: not always true, but commonly).
An example might be a DB of cars - on first glance, you might think that the licence plate could be used as the key. But these could be changed so that'd be a bad idea. You wouldnt really want to find that out after releasing the app, when someone comes to you wanting to know why they can't change their number plate to their shiny new personalised one.
