What field pays more in salary ..medical or astrophysics?

Medical, by far. Even the lowest-paid doctor still makes 100k or more; that's about the highest paid astronomer, and you're often looking at a starting salary of 40-50k right out of grad school with a PhD in astronomy or physics. However, they can both take about the same amount of time in school/training, and astrophysics grad school pays YOU to get a PhD so that would be cheaper. But you really need to love the field you're doing, or you'll hate your job no matter how much it pays.