Comparison of IntelliJ Python plugin or PyCharm

Since this question (and its accepted answer) are older and new versions of both IDEA and PyCharm are available, I figured it would be appropriate to add a new answer without modifying the existing "correct" one...

My company has licenses for both PyCharm 1.5 and IntelliJ IDEA 10.5 and I have used both for regular Python development and Django web app development. From my experience, I have not found any features of PyCharm which were not available to me using the Python plugin for IntelliJ.

This makes sense if you consider the fact thet the Python IntelliJ plugin is actually maintained by the PyCharm developers at JetBrains. The only catch to this is that there could be brand new features released to a new version of PyCharm which would not be immediately available to the plugin users but it probably wouldn't take them long to port them over.

If you are considering buying a license for IntelliJ, I would recommend to not also buy a PyCharm license as it seems like a waste of money. However, if you only do Python related development, I would recommend to only buy the PyCharm IDE instead because its licenses are cheaper than IDEA's.

UPDATE (2013-01-23)
I have been recently using IntelliJ IDEA v12 with the latest (2.10.0) python plugin instead of PyCharm. As an experienced IDEA user, this is my preferred situation. For someone with no (or less) IDEA experience, JetBrains actually recommends starting with PyCharm because it's UI is simpler and more focused on productivity within Python projects.

As for feature difference, JetBrains claims that the Python plugin is built from exactly the same source as PyCharm so the features should be identical. The only differences should be with the user experience, where IDEA's plugin is slightly harder to use because that IDE isn't aimed at Python development in the first place.

JetBrains has a FAQ that has more info about differences between the IDEA plugin and PyCharm.

Right now PyCharm is quite a bit farther ahead in terms of functionality compared to the Python plugin for IntelliJ IDEA 9. Once we start the EAP for IntelliJ IDEA 10, we'll also release a new version of the Python plugin containing all the latest features of PyCharm, but we don't currently plan to backport the new features to the IDEA 9 version of the plugin. So, if you're OK with using EAP builds of IntelliJ IDEA, you shouldn't need to buy a separate license for PyCharm.

IntelliJ's comarison of IntelliJ IDEA and PyCharm.http://confluence.jetbrains.net/display/PYH/PyCharm+vs.+IntelliJ+IDEA+Python+plugin+FAQ

From their blog:

Nov., 22nd 2012 - Just a quick heads up: we’ve just released a version of the Python plugin for the latest Early Access Preview build of IntelliJ IDEA 12 (build 123.4). The plugin includes the entire functionality of the current PyCharm 2.7 EAP, including several features which were previously only available in PyCharm (creating virtualenvs, using remote interpreters, creating UML diagrams for Python code etc.).

Source: http://blog.jetbrains.com/pycharm/2012/11/python-plugin-for-intellij-idea-12-is-now-available/