Welcome, Gran23. Reallistically, stabilily is personal thing. It kind of boils down to your preference and use. If you're not really concerned about paddling long distances and plan on cruising at 3mph or less, go with the 31" bottom. More room and probably enough stabilily to stand comfortably. if standing is not on the list and room is not a big necessity, go narrower. I can promise you that any bit of tippyness you experience will lessen with use. There's no magic number for stability, speed, room, handling, etc. Short, fat boats are more stable , slower and more manuverable and long, skinny boats are more tippy, faster and a bit harder to turn.
My pirogue is close to the UJ in size and it as good an all around boat as you can get. Unless you need a cargo hauler, go with the 27"bottom ( or less).
Joey