Longer PhD with a deeper result vs a shorter PhD with a sufficient result
There are several considerations regarding whether to graduate now.
I'll go through them from most to least important (in my opinion).


Thesis readiness:
You can't graduate unless
your advisor and your committee feel your thesis is good-enough.
For some departments, you need to have 2 or 3 separate projects
which are in advanced enough state or even published.
Research readiness: As @seteropere noted,
if you feel that you are able to do research independently,
and your advisor agrees, then you are ready to graduate.
Future job situation:


If you can get a tenure-track job or post-doc in September 2015,
then go ahead and graduate in June 2015.
If you can't get any academic jobs in September 2015,
then maybe you should stay one more year.

Job prospects this year vs next year:
For some fields, getting a good job requires a student
to have papers in advanced stages of the publishing pipeline,
i.e. submitted is good, under second or third round of review is better,
and accepted is best.
If you have a great paper but it is not yet ready,
waiting until the next year before you apply for jobs
may allow you to have that great paper
in a more advanced stage of the publishing pipeline
which can't hurt your job prospects.
Funding from advisor:
Is your advisor willing/able to support you for an additional year?
Some advisors limit their support to 4 or 5 years,
and students who wish or need to stay longer may have to carry heavy teaching loads.
Research environment: Are you learning from your advisor,
and doing good work with the other students/professors in your current institution?
If so, leaving for another position becomes more risky
because the situation may be worse than your current situation.
Financial situation:
Usually, a PhD salary is less than a post-doc salary,
which is less than a tenure-track job salary.
For some students who may be supporting a family,
earning more money as soon as possible is an important consideration.