Well, if you are using Spring you may as well use the whole stack, since it is so mature.
Also Spring Security gives you not just HTTP, but method-level security as well via @Secured annotations.
Also, I am not too crazy about the Dropwizard requirement of having to explicitly code an @Auth object into every REST method.
It seems that a careless programmer could easily forget about it and leave an unsecured method open.
The filter-based Spring Security approach ensures every method is equally protected IMHO