Stupid question on INDEXES

I'd like to know if i think in the right way. With
CREATE TABLE Test1 (
my_id int,
descr text,
primary key (my_id))
Postgres automatically creates an index named test1_pkey.
If i use
CREATE TABLE Test1 (
my_id int,
descr text)
without specifying a pkey, i can write
create unique index i_test1_pkey on (my_id).
The only difference is that in the 2nd case i can use the name i want, or
the usage is different?

Many thanks

Giorgio