After borrowing a friend's last year, we just purchased the Topeak Baby Sitter II for our bike. Our daughter absolutely loves it and I feel very safe with her in it. The new version also accomodates children up to 48 lbs, more than other bike seats. It is a bike seat, not a trailer. The trailer may not tip if you do, but I worry about city biking and cars just plain not seeing that trailer. If you have a good quality bike seat and a helmet that fits properly, your child will survive if your bike tips. In fact, my friend fell with this bike seat and her 1 yr old didn't even cry! We got it for $99 on Ebay and you can also get a diaper bag (the Topeak Mamasan) that clips onto the back of the seat (no backpack in baby's face). After your child is too old for bike seat, you can use the rack that comes with for groceries, bags, etc.
I ruled out the Ibert simply because I thought it would be difficult to steer, to see where I was going, or to do any kind of serious bike riding. Probably fine for riding to the park...
Happy biking!