Re: Relevance
If you really don't think you'll do any more maths/maths-based subjects (be it higher level financial/engineering), I'd probably go for psych. The stuff I learnt in specialist maths and calculus 2, I'm pretty sure I've never applied since leaving those subjects. Even the stuff I did in a stats subject was more linked to Maths Methods material (though if you were to do very high level probability or statistics, that could well change and the Calculus 1/2 material become much more relevance in proofs).
On the other hand, the stuff covered in psych (which I believe to include areas of the brain, and other relevant material about behaviour) is pretty important knowledge. That said, it's a lot easier to slot into a second year psych subject without the first year subject than a maths subject. Also, I believe you do cover some brain in anatomy - though I'm thinking it's not to the same extent, and without a lot of the additional details you'd get in a psych subject. That might really work as an argument for psych, if you're hoping for additional reinforcement between courses.
Re: Easiness/Marks
If you're kind of decent at maths, you'll find calc very easy. If you didn't enjoy methods much, psych will be easier. Easier in my view to get a very high mark in calc (just due to the nature of the assessment - no essays in maths), though I found the one psych exam I did quite straight forward.
Were I not confident with calculus, I think it would've been easier to come out of the psych exam feeling happier than coming out of the calculus exam. Not sure if I'm making it very clear what I mean! Calculus - content is on average a bit harder, but everything that is examined can be learnt, so getting a very high mark is easier. Psych - content on average a bit easier, but there may be some ambiguities (both in the exam, and in the subject nature of essay marking), so a very high mark not so easy.