I am a Rubyist that is just starting to dabble in Smalltalk and am trying to chose an implementation to experiment with. It seems like Squeak or Pharo would be a good choice, bu
There might be a clear favourite but you have to develop it for yourself.
Pharo and squeak have huge parts in common. But as pharo is a fork from squeak it also means they diverge from each other.
If you look from the angle of learning smalltalk then both are equal. If you look from the angle of supported technologies then the situation might be a bit different. Most developers are either squeakers or pharoers. And developers create libraries and technology supporting frameworks. If the developer of a library is a pharoer I would expect the library to be better maintained in pharo than in squeak (and vice versa).
You should probably have a look at both and try the stuff you want to achieve. In most of the cases the experience will be the same. As soon as it stops being the same you might know your favourite already.