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From what I see, there are examples where both 洞 and 口 can be used to describe holes in things. For example from samples in Pleco:
衣服撕了个口 (A hole was torn in the jacket)
衬衫破了一个洞 (Have a hole in one's shirt)
This being the case, what are the differences in usage between these two characters? Do these relate to different types of holes? Is one character more common than the other one for this use case?
In my opinion, this is a "口":
This is a "洞":
口/口子 means "(人体或物体的表层)破损的地方[the place where it is damaged(on the surface of a human body or an object)]", 洞 means "物体中间的穿通的或凹入较深的部分(The penetrating or deep recessed part of the object)". You can say "我的手破了一道口子", but absolutely cannot say "我的手破了一个洞"! Or I can't imagine how panic the person who hears it will be.
Some other examples:
日子久了，水滴竟在石头上滴出一个洞来。(水滴石穿)
他的手被小刀划了一道口子。
"洞" generally means larger and deeper holes; "口" generally refers to small and shallow holes.
