Database vs File System
There are no real advantages or disadvantages to using one of these storage types over the other. Keeping items in the database provide an easier backup method, while using a file system may be slightly more efficient as there are less queries that need to be run. But both require disk space and are easy to manage via the AdminCP.
At the end of the day it comes down to your own preference on how you want to manage your user picture images and how you want to store them.
The default storage type is to store the images in the database.