I think the two folks above covered it pretty well, so I'll merely add that i is not an either/or choice. I have both types because for certain jobs, one type is preferred over another.

For most of my regular activities, such as a quick run for groceries, or dropping off the drycleaning, I love the compact, easy-handling, lightweight monowheel, but when I've got to move heavy stuff or lots of stuff, I go for the dualie.

The mono is the best for threading through downtown congestion, while the dualie shines when it's time to haul the cooler, a blanket, wine, the soccer ball, and lacrosse sticks down for a day ar the park.