The UoL course looks harder - unless you purposefully chose a load of higher level maths modules at the OU, beyond the requirements.
It also looks like the maths and economics sides of the course are well integrated at the UoL, whilst at the OU maths and economics are entirely separated. For instance MST209 teaches about PDEs and advanced-ish calculus within the context of Newtonian mechanics, and the OU statistics courses talk a great deal about medical statistics, animal populations and such like. The OU's main level 3 economics course (dd309) doesn't seem to have any mathematical pre-requisites at all.
Then again, the OU materials are very well designed and well written for use in self study, they have a lot of experience of providing distance study courses, and there is a lot of support both from tutors (at tutorials which would be about twice a month for full time study, and via phone/email) and from fellow pupils (via busy internet forums). To a certain extent it sounds like the UoL just throw textbooks at you and expect you to get on with it, which could be tough and lonely (though I don't know having not studied there). Plus at the OU you have a qualification with every module you pass, should you decide to go back to full time work, or to move to a different university. It looks like the UoL gives you nothing until you finish the whole course.