Choosing primary key type: 64 or 52 bit primary keys?

Hi all,

I'd like to use an integer number for my primary key. I need it to be
bigger than 32 bits.

As far as I understand I have two options:

a) use all the 64 bits of a 'bigint'
b) use the 52 mantissa bits of a 'double precision'

My question is, which would be faster for indexing? I assume the
bigint wins here, right?

Thanks in advance,
Antonio