I'd say that it really depends on what the code needs to do.

For example, if the code is to be distributed and extended as a third-party library, then the class hierarchy is probably a better fit to allow extensibility.

But if the purpose of the API is to compute the area of given shapes (as in the example), then it makes sense to make it efficient and there is no use to provide extensibility to the outside world.

The advocates of "clean code" that Casey mentions will go for extensibility no matter the use.

I find it very interesting to have numbers to weight what you are leaving when you go the extensibility route instead of the non-pessimistic route.