One big difference: PA requires about 1000 hours of hands-on (read: paid) work with patients. OT requires observation hours. A lot more work on the front end to enter the field of PA.
However, if you're looking just at the numbers, you're going to make more and the job prospects are just as good for a PA. My guess is that it's more stressful (unless you're working as an OT in a high turnover setting). I like alansda's point about therapy vs. medicine: I've found that in medicine, mid-level (and especially high-level) professionals are being pushed to always consider the bottom line. You see this in OT as well, but it is exaggerated in medicine. For example, many (if not most) hospitals run off of a business model (as opposed to, say, a care model). You are expected to perform so many procedures, push so many prescriptions, etc...and if your fail to achieve your outcome (i.e. the patient stays sick, or if something goes wrong), you could be sued. I am speaking from what I know of family who have worked as mid-level practitioners, and this was 10 years ago. I'm sure OT faces some of the same challenges, but the difference is, you're working to help people achieve some degree of function in their lives (or assessing/working with them to set down goals). In other words, you're almost expected to not view people as numbers, as opposed to doctors or PA's who are almost expected to do the opposite.
I'm not blaming the actual practitioner (although sometimes, it's just simply their own narrow-mindedness), but the way these things are run doesn't lend itself to getting to know people as people. It sounds simplistic, but the people on here who advise pre-grad students to choose based on their interests really mean it. The field needs people who can handle the pressure of high-volume and risk associated with being a doctor or PA, and it also needs caring people to spend time helping people rebuild their lives. If it's just about the money, it would really be worth it to just become a doctor. Otherwise, I'd just take inventory of my interests and go from there. Good luck!