Psychology from an objective standpoint is easier. I'm currently accelerating both 3/4 Bio and 3/4 Psych at the moment, and psych is for sure easier. There is far less content to memorise, and it is easier to understand the language surrounding the theory. It is also fairly applicable to real life scenarios and experiences everyone has had, so it's far quicker to be able to consolidate the information by relating it back to your own experiences.
Biology on the other hand is far more content-heavy compared to psych, and I'm surprised it doesn't get scaled more. I find 3/4 bio harder than my chemistry class (although maybe that's because it's only 1/2). Biology is surrounded by a lot of technical jargon which takes quite a while to understand and it is not nearly as easy to consolidate the information using real life examples.
So, I'd say that if you want the harder route with more technical stuff that makes you sounds like a nerdy scientist, biology is your go. Psychology is easier to understand, and if you want to focus on other areas that are more technical, psych is reasonably laid back so long as you pay attention to what is being said.