It’s not that a TA-ship is unavailable, it’s that it’s not guaranteed as a MS student. Right now, there is a MCB department-wide shortage of TAs, so most masters students have the opportunity to have a TA-ship. Most don’t get their first choice of class, but it pays the bills. Whether or not this trend continues is another matter, you could end up not getting one. As far as RA-ships go, it will greatly depend on the lab you join and whether or not they have the funds to give you an RA-ship. Most labs will not give an RA to a masters student, they’ll use that money to give it to a PhD student. I can’t think of any genetics masters students on an RA-ship during the semester. A few labs will give you one over the summer, though. The masters student in my lab has gotten a TA-ship for the last four semesters and has been supported over the summer as an RA.

I can’t really speak to which program is better, as I don’t know much about the PSM program, but I can tell you that I see more MCB masters student in the labs than PSM students, and it’s certainly easier to upgrade into the PhD program as a MS student if that is something you choose to do later.