They have different purposes, or different tradeoffs. When my older son was small enough, we used a stem-mounted seat (that is, near the handlebars) for cruising around and other little rides, and a Chariot trailer for bigger rides (I race bikes, and he's done a lot of training miles with me). They're very different. In the seat, we could talk, hold hands, he could pretend to steer, ring the bell, etc. In the trailer (which he still sometimes uses as an alternative to his Trail-a-Bike, at 3.5), he's warmer, can have some toys and snacks and he can fall asleep (actually he fell asleep in the seat a couple of times, but I didn't love that), but we weren't as "together". When my newborn is old enough, we will use the seat and the trailer the same ways again. But my wife (who is an experienced cyclist by any normal standard, but not of the 1,000-hours-a-year variety; she's also smaller) was less comfortable with him in the seat than in the trailer.