Question about backups

[Chiron]

Good afternoon

I've never used the backup feature of xbian but I want to repair this mistake.
I would use "file" as type of destination.

What is best:
- Use a usb key connected to the raspi? (what format would you recommend for it ext2? ext3? ext4? another one?)
- Use a network share on my nas? (My raspi is connected to the network through wifi).

Thanks. :-)