Only storage, as ram usage will benefit from minimal templates.
Less code running on a vm means less code that can potentially be exploited!
You should look into an apt-cacher-ng qube: it replaces the default proxy used for updates and caches the packages, so you only have to download them once for all your templates.
These notes are enough to get you started: https://github.com/unman/notes/blob/master/apt-cacher-ng