Postgres unique constraint vs index

As I can understand documentation the following definitions are equivalent:

create table foo (
    id serial primary key,
    code integer,
    label text,
    constraint foo_uq unique (code, label));

create table foo (
    id serial primary key,
    code integer,
    label text);
create unique index foo_idx on foo using btree (code, label);    

However, a note in the manual for Postgres 9.4 says:

The preferred way to add a unique constraint to a table is ALTER TABLE ... ADD CONSTRAINT. The use of indexes to enforce unique constraints
could be considered an implementation detail that should not be
accessed directly.

(Edit: this note was removed from the manual with Postgres 9.5.)

Is it only a matter of good style? What are practical consequences of choice one of these variants (e.g. in performance)?