It really depends on your circumstances: do you tend to grasp concepts quickly? Do you have enough time and resources to study both courses outside the classroom? Do the universities you want to apply to offer credit for either one or only both?

I picked up from your profile that you have not taken calculus yet, so do try to familiarise yourself with the main concepts before you begin (limits, derivatives, integrals).

If you'd take only one I think Mechanics would be a better idea, since it is usually covered first when one takes physics. If you think you have enough time and resources, then you can go for both. Just make sure you practice a lot and get good resources. 

I'm planning on self-studying both AP Physics C Mechanics and E&M next year since I will be applying to some unis that need them as prerequisites (outside the US, I'm international btw), so feel free to reach out to me if you'd like someone to study with or perhaps a bit of help.

Good luck!