I once assessed Arden vs OU for Computing by comparing the details of the module descriptions in Arden's PDF brochure vs the OU's website. That's admittedly limited info to base a decision on, but my conclusion was that the Arden course seemed quite basic. No maths, very little programming, etc. Lots of focus on the business side of things rather than theory or technical detail. Really not what I was looking for.
Psychology might well be different though. I'd encourage you to get the relevant Arden brochure (they want an email address, but you can use a throwaway one like 10minutemail) and compare the details to the corresponding OU course.