In the end, I decided to the triangular pocket (looks more like a boot to me, actually).
The shape will prevent whatever I do put in there from ending up above the pocket opening and subsequently falling out of my pocket. I lose enough stuff on my own without any additional help from gravity, thank you very much.
My goal is to have this finished by the weekend, so that I can begin a new project. What will it be? Perhaps a new hat-and-mitten set to go with my new coat. All of Sandy's mitten knitting has been inspiring. Perhaps the BPT hooded cardi that I blogged about earlier. Wait and see. I'm not being intentionally cagey, I really haven't decided yet.