Here are a couple of things I would think about
:
-Definitely the quality of the certifying agency
-Do you like the Y? (I have to be in a gym I love. There are some places that I would loose my momentum if that is where I had to go. KWIM?)
-Do you enjoy group instruction?
-Obviously personal training tends to be more lucrative, buuuuuut you could study for that while teaching the group classes.
-Have you looked into the Y's PT program? Do they offer certifications for PT? A lot of gyms will have like all day Saturday trainings. If so, maybe you could get a discount if you already were an instructor there. It never hurts to ask.
In the end, if you are wanting to really make a "profession" of this, you are going to need both. If it is just something you want to do on the side, just figure out which you prefer more.
Good luck with your decision! You will be AWESOME at either or both!!!