Obviously, this depends on what you're stronger at, however, from personal experience I'd say psych is probably easier to do better in. If you work hard enough and dedicate yourself to the subject, it's possible to do really well in either subject. That being said though, both subjects are popular and thus are quite competitive. With this in mind, for different reasons it can be difficult to score highly (i.e. 40+) in both subjects and as it is with most subjects, you need a deep understanding of the concepts to succeed. Whilst the mathematical concepts aren't overly difficult to grasp (this is coming from someone who is relatively average at maths) in Further, the work can, at times, be rather tedious and lead you to make simple mistakes which are very costly. Psychology, on the other hand, is quite content heavy, though the content itself isn't too difficult to understand. For me, I found that the content of psychology was easier to understand because it mostly related to real life and 'made sense'. I ended up getting the same SS in both classes, and I worked a lot harder in further than psych (obviously this statement is to be taken with a grain of salt due to individual circumstances).
Ultimately, it all comes down to what you think you'd be better at and what you would enjoy more (I cannot emphasise how much easier it is to do well in a subject if you enjoy it!). I'd stick it out for the holidays and do whatever homework you've been required to do and see how you like that. In addition, you can look at the study designs and exams from this year (you can probably find the exams in the '2017 Exam Discussion' board) and try and determine which subject you'd rather do, asking any friends in psych how they find the class/teacher could also prove to be helpful too. Whatever you decide, good luck with it all and don't stress about it too much in the meantime (easier said than done, I know)!
(Edit: I've just noticed your other subjects so please bear in mind my comments are written from someone whose personal strengths lie in english/humanities type subjects)