I have worked with external and internal to the large extent .
External pyramids are little bit slow then internal one's because the data has to be ready from the external file . GDAL has to read .ovr file.
Pros of external overview:-
- Less space to store.
- Handy and can be deelte anytime if not required.
Cons of external overview:-
- Less Speed
- takes 15% more size w.r.t. actual data size on hard disk.
Pros of internal overview:-
- takes 35% more size w.r.t. actual data size on hard disk.
- once created you can remove internal pyramids but space used for internal pyramids will not come back. Ex:- if 300 MB original dataset , we build internal it becomes 415MB , when we remove internal pyramids, size of raster will remains 415MB.
- can only be build if data set size don't exceed 4 GB
Cons of internal overview:-
- Faster Speed.