1. auto purge when any plugin updated as you suggested
2. has a quick purge as a sticky top bar on any admin screen. so you can easily purge all.
I would prefer option2, as option1 may not necessarily mean you have to purge all. for eg. when you upgrade a plugin, which does not affect any page content.
What do you think?
re. option 1. It's important to note that we don't really know how a plugin update will affect the page content/site layout. Even though a plugin could be unrelated to page content, still doesn't guarantee that an update won't affect page layout. That is why I think the auto purge on any plugin update would be an important feature.