I'll try my best to help you decide between both of them but its going to be tough as I take both of them as higher level subjects and love them both.
Psychology
It's an extremely interesting IMO. You learn about the biological/cognitive and sociocultural level of analysis. I've done the biological level of analysis so far so I can only tell you about what that level of analysis involves. You basically look at how different things (neurotransmitters, localisation of function, hormones, genetics etc) affect/effect behavior. As well as principles of the biological level of analysis, research methods that are used etc and ethical issues of studies that have been conducted to look at stuff to do with the biological level of analysis. It's an extremely essay based subject as the exams (paper 1 and 2 - what you'll be taking as a SL student) are entirely essay based. So if that doesn't appeal to you I'd reconsider as economics in that respect is much less based on how you write your essay but what you include in your essay - psychology essay a bit more like English were you need to master the structure and content to get marks.
Are you a person who's more interested in things like biology, the brain, how people function etc or are you more interested in logical things (like maths - calculations etc)? That is the main difference IMO between psychology and economics. Psychology would probably appeal more to artsy students (which you seem like you are as you're taking higher level languages?) where as economics would appeal more to people who are into logical thinking - so maths, sciences, that sort of stuff.
EconomicsEconomics is equally as interesting as psychology but it tends to get dry sometimes especially if you're teacher lectures in a monotone voice - then its a nightmare! You essentially learn about how the economy works in allocating scarce resources efficiently - you look at things in small scale (microeconomics - firms, individuals, governments etc) and the large scale (the economy as a whole). Economics is also an essay based subject like psychology but you aren't marked on your essay writing techniques but rather on the content of your essay, so the key to writing economics exams is to jot down everything that is relevant rather than worrying about how its structured etc as know one is marking you on that. Economics has a mathematical component (although you don't have to take the qualitative paper as you're a SL student) - so if maths isn't your strength then perhaps take psychology? Or choose according to the subject material you're more interested in learning?
I have to mention that economics is an easier 7 (around 12-13% of students get 7's in SL economics) where as psychology is pretty damn hard to get a 7 in (around 4-5% of students only get 7s). Plus psychology is much more memorization and work intensive where as understanding the concepts in economics is necessary rather than memorising a bunch of studies. That brings me to my next point - IB psychology isn't like what you read on magazine and what not, its all about looking at different concepts and learning studies that investigate those subjects and linking the findings of the case study/experiment to the central concepts. So you'll be spending a lot more time learning case studies/experiments than learning about the things you read in magazines. -> If that doesn't interest you do economics. From the perspective of universities, economics is a well respected subject but psychology is seen as more of a 'soft' subject so to speak - this may tip the scales for you if you're interested in more competitive universities BUT UK universities care much about your SL subjects so it probably won't make much of a difference anyways. I personally recommend you take economics but its up to you.
Hope I helped! Feel free to ask more questions