I'd say go for Maths.
Bear in mind Medicine is highly competitive. Choosing Maths as one of your choices will boost your application, as it is considered a strong subject.
Obviously you can still get in with Psychology, but bear in mind you'll be competing against candidates who probably achieved AAAA at AS level in Maths, Chemistry, Biology and probably another strong subject such as a language, history or physics.
However, don't do maths if you think it'll be too much work. You should choose it if you believe you can at least get a B at AS.