They are the same, it is just a matter of preferences.
I also asked myself why would we add this new syntax if we already have one? Well, Programming with Ruby implies that we are lazy and want to type the less possible caracters. So this new syntax allow us - lazy programmers - to write the same thing, minus 1 caracter!
But keep in mind some stuff, like the type of the keys for instance (Ruby 1.9.3):
> {a: 12}.class
=> Hash
> {:a => 12}.class
=> Hash
> {'a' => 12}.keys.first.class
=> String
> {a: 12}.keys.first.class
=> Symbol
Also, some declaration are illegal with the new syntax:
> { '1-2' => "something" }
=> {"1-2"=>"something"}
> { 1-2: "something" }
SyntaxError: (irb):38: syntax error, unexpected ':', expecting tASSOC
{ 1-2: "something" }
^
(irb):38: syntax error, unexpected '}', expecting $end
For more informations: Is there any difference between the `:key => "value"` and `key: "value"` hash notations?
