Go with the trailer. At first glance, a trailer may seem more vulnerable because it adds width to the bikes profile and your baby seems far, far away from you, but study after study shows that a trailer is safer. Heres why: A bike seat raises the center of gravity, which can result in some spectacular crashes, often involving head and neck injuries. Plus, as your child gets older and starts throwing around her weight, it will be harder to maintain a neutral position as you navigate traffic around town. If she should fall, it would be a long way to the ground.
A trailer, on the other hand, is a lot more stable and affects the handling of the bike a lot less. Added bonus: It also has room to pack plenty of toys and snacks that will keep your kid entertained longer, which gives you freedom to finish your errands.
Burley (burley.com) has been in the bike trailer business for 30 years and has been heavily involved in setting the American Society for Testing and Materials (ATSM) safety standards for bike trailers. Their new for 2009 D'Lite ST ($599), allows you to bike or stroll with your baby and includes super-functional features, such as removable, washable shoulder and harness pads, cargo space for up to three bags of groceries (can haul a max of 100 pounds total), and a quick-setup tow arm so you can be off and cycling in a matter of minutes.