The reason for the extra year is that Scottish universities allow for more breadth: in the first two years you will be able to study up to three subjects, and it's entirely up to you beyond your department's basic requirements. So, in first year you would take economics 1 and 2, but for example you could also do Spanish and management. The Scottish system allows students to broaden their education, which some appreciate, whilst others prefer for the more concentrated English approach where you would solely study economics.
In my first year, I took chemistry, maths and biology, but my friend took physics, maths and medieval history; another took English lit, sustainable development and music. In the end, some students actually end up changing their degree path if they find something else they prefer!
Another aspect to consider is the student experience; Exeter is a big English city, whilst St Andrews is a small Scottish town. I don't know much about Exeter I'm afraid, but if you have any questions about St Andrews, please ask. I'm actually going back into first year as an economics student due to personal reasons meaning I've had to leave early this year, so I'll maybe see you in September!