I would expect both "open source" code to be of approximately equal quality to proprietary code. In each ideology you will get people who care (about quality), and people who don't, in approximately equal proportions, the same with skill, ingenuity and passion for the work.
The difference is that proprietary software is constrained by the number of developers able to view and work on the code. An open source project may have a similar number, or smaller set of core developers, but a much larger pool of developers that can spot problems, suggest alternatives, fix the one bug that is affecting them, etc. Having a more diverse set of developers will increase the chances that the software improves.
You could also make an argument about the motivations of the developers. Open source projects are often a community of people passionate about what they are building and have a strong incentive to make their code readable by others. By the nature of open source a developers reputation is on the line with every bit of code they make public. I've met far more developers scared to make their horrible code public than those worried about getting fired for equivalently horrible code.