No differences in SQ. There may be a difference in the behaviour of Watched Folders between local storage on the Nucleus and NAS storage.
Watched Folders on the Nucleus are watched in real time. Some NASes do not always report changes in their storage, so you may need to set up a scheduled rescan interval for the NAS. See:
As for advantages for internal and external drives on the Nucleus, it’s down to personal preferences. An internal drive is neater (no external boxes), but if the drive needs replacing, it’s quicker with an external drive. An external drive can be formatted so that it can be plugged directly into a Windows PC or Mac. Copying files to/from the drive is then quicker because they are not travelling over the network.