The more important point is that if you're starting out with Python, which one to use is very clear. There's Stackless and so on, but the main page at Python.org just points to CPython. (It does mentions that Python is available for the JVM and .NET vm, but that's not the primary link.)

It's a sharp contrast to when people have to decide which Lisp or Scheme to learn with, but don't have enough experience to choose. (Some probably decide on Python instead.)