ext_icon.svg should be preferred
What do you suggest?
If available, show the SVG icon. If not, fall back to the GIF version.
The TYPO3 Backend's Extensionmanager do this since a long time. This Icon-type is modern and more beautiful.
Add an use case
Test it with the extension Mask. It delivers both graphicformats: /typo3conf/ext/mask/ext_icon.svg /typo3conf/ext/mask/ext_icon.gif