When I buy rear lights, I make sure they have mounts for both seat post and seat stay. I use both. If you just use one on the seat stay, make sure it is on the side facing traffic and as high as possible.
I would never put a light on a seat bag, jersey pocket or backpack. Those lights are almost never facing the proper direction and are often not seen by approaching traffic. I use Cygolites, which come with both seat post and seat stay mounts, but many other brands offer them as well.