And yet if your goal is to learn about OO programming from the concepts of its OO model, then either language is good and Ruby is more convenient.

On the other hand if your intent is to learn about its collection classes, what MVC looked like in its original inspiration, and so on, then you should learn Smalltalk.

There is more to learn than any of us has time for. If you know what you hope to get out of learning something like X, then you should pick the most convenient X around to learn it.