Database vs File System
There are no real advantages or disadvantages to using one of these storage types over the other. Both require disk space, keeping items in the database provide an easier backup method, using a file system may be slightly more efficient as there are less queries that need to be run and both 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.
Note:
The default storage type is to store the images in the database.