P2 setting is MXF files
This is more of a what works best for you workflow.
Now that version 4 of the firestore software makes a ref movie or the quicktime files, they are easier to handle long clips.
My question for you would be to you want to log your footage in a tradition manner, so you name unique names and description on transfer. Also wanting to trim and view the footage before getting it into FCP. – Useful for documentary and film type work. Then P2 is better.
If you film long clips and you just want to bring in the entire clip and start working on it then – concert footage then Quicktime maybe best.
One thing to remember is always make a backup copy of the footage. The p2 method more or less force you to do this, while you can (but shouldn’t) bypass it with the QT method.