Both are underpaid. Most professors are underpaid. If making more money is a motivating factor in becoming a professor in either math or physics you will probably be disappointed.
Salaries for new assistant professors range on average from around $60k for philosophy to $120k for finance and $127k for accounting at AACSB schools. I'm not sure where math or physics fit in or if there are any others outside that range. Most subjects will be much closer in salary to philosophy than business. These numbers are 9 month salaries, but the subject will also largely determine whether there is opportunity for any additional summer funding (don't count on it).