My advice would be to look into the courses and course description of the classes you'll have to take for Applied Physics and Mechanical Engineering. Look at each and compare them. The classes might give you a better understanding of what you'll be getting into before you invest in a major. If one is not posted online, I'm sure you can email an academic counselor at each college and ask for a list of the classes. Each school you apply to will be slightly different. In Engineering will make you cover a broad range of materials besides your typical high school classes, you'll get into course work such as: thermodynamics, material science, 3D CAD software design classes, etc. Physics will be more restricted within the physics and math realm, you'll probably take classes such as: Quantum Mechanics, Vector Calculus, Matrices, etc.
Overall after college, I'd say physics most people either go into teaching, grad-school, or enter a job that is related to a lab setting. You'll be doing a lot of research and 'lab coat' type of work. Generally the people I know with Science degrees peruse higher learning and go to graduate school. With mechanical engineering, people generally work right after college and some go to grad-school. You can do research and lab work in engineering as well. In mechanical engineering the more popular sectors to get jobs in are Oil&Gas, Construction, and Manufacturing/Design.
I hope I answered your question!