It depends on what state you live in. When you are involuntarily admitted by your doctor, police, E.R., and so on (meaning you have zero choice--you HAVE to go) it's not so much as you are treated differently while you are there...But they keep you as long as they see necessary. Usually a 5150 hold is a mandatory 72 hour stay. You can search 5150 hold online to see how your state specifically handles it.
It is worth looking into it either way because there are certain rights you have once you are there but they won't tell you that.

If you check yourself in voluntarily, you still really have no stake in deciding when you will be released, but doing so may avoid the hold. They will only place you on the hold if you have demonstrated that you are a harm to yourself or anyone else.

It's much better to stay without the hold. When I went it was voluntary and I was there for a little over 48 hours; but there were others who were involuntarily admitted and some had been there for over a week. Whenever I asked if I could be released or reevaluated because I "felt better" (I was super homesick) they began threatening me with a hold.

Okay now that I've shared that info and my experience....Take a deep breath and know that everything will be okay. Do what you feel is right. It is important for you to feel safe. It is a scary experience but not as scary as the thought of doing something irreversible.