Now that I'm
This recommendation isn't based on thinking PyQt is significantly better than PySide: they are roughly the same packages.2 That said, PyQt has a much more active user community, with some amazing developers, such as Phil Thompson, who quickly address bugs and other serious issues. PyQt has kept pace with Qt's version 5, while PySide is still tracking Qt 4. Nokia used to actively maintain PySide, but my understanding is they have dropped it, so there is now no longer a cadre of professionals working on active maintenance. There seem to be lots of bug reports piling up, for instance.
In my experience in the open-source world, when it comes to things like navigating install hell, finding out if something is a feature or a bug, or learning how to optimize code, community is invaluable. There is a large body of know-how that only emerges if a lot of people use something. For that reason alone, I recommend beginners use PyQt to cut their teeth.
Also, PyQt has much better documentation. I have found, countless times, for some reason PySide docs leave out crucial details about methods and classes that PyQt includes. Not sure why, but the automatic documentation generator is much better for PyQt (for one representative example, compare the online docs for QTextEdit.setAlignment method for the two frameworks).
There is another good reason: Summerfield's book is the best introductory book on Qt programming in Python, and it uses PyQt. It rescued me me from floundering as a Qt cargo cult programmer, where I would just hunt (Google) and peck out something that sometimes worked. His book teaches the fundamentals of the framework from the ground up. Admittedly, the PySide Summer repository does take the sails out of this reason a little bit, as it should make it easier for beginners to work through his book with PySide. (Yes, this entire post is tinged with irony in a few ways).
The main exception to my argument is going to apply to a tiny fraction of beginners. Namely, if you are working on a commercial product, and don't want to pay a licensing fee, then you should use PySide. The reason PySide exists in the first place is that PySide has a less restrictive license than PyQt. If you want to use PyQt for commercial software, you have to buy the commercial license from Riverbank Computing. This restriction does not hold for PySide.
Note many people start out with dollar signs in their eyes, but in practice if you are building a little application, and are truly a beginner, you could easily build in PyQt first while learning. If you want to commercialize the product you could easily port to PySide.
I should be clear: my argument isn't that PyQt is objectively better than PySide, but that because they are basically the same framework, for beginners I'd suggest going with the package with the most active community.
1 That is not to say the documentation is fantastic for PyQt. For someone neutral about C++ versus Python, I would recommend starting with Qt in C++. The documentation is incomparable, the number of books you can get, etc.. This post is focused on Python, as between Qt and PyQt, there is really no contest.
2 They are roughly, though not exactly, the same. The PySide binding is built using Shiboken while PyQt uses SIP. This can be a nontrivial difference, as SIP is more mature than Shiboken. Plus, PyQt is on version 5, as mentioned. The differences, as of Fall 2014, don't seem significant enough to be deciding factors. (Note added in early 2015: it is clear this will become a larger factor as time goes on. So far there is talk, but no palpable progress, in moving PySide to Qt 5).