i have only read the first page of responses to this, but it seems a little charged. i thought i would put my two cents (and sense) in.
i have ridden kids on bicycles in many different fashions. as we were car free for many years, and still aim to use a car as little as we can, it is pretty much the only other way to go. in our time, we have had many different set-ups. with a tiny little baby (starting at one month old) and a 3 year old, i rode a tricycle with my baby "moby"-ed to me, while my oldest was strapped into her pillow filled basket seat. we have used front seats, back seats, bike trailers, and an ergo (all on a two wheeler) as well. i would have to say that in my years of every day bike riding, the ergo is way safer then a back seat, which throws your balance way off. my personal favorite is riding with my little one in a front seat, which is much more centered as far as weight distribution goes. i remember strapping my 3 year old in an ergo while h e a v i l y pregnant with my (soon to be) middle kiddo.
how one rides really depends on what their personal comfort levels are. if i had a choice between an ergo or a back seat, i would say ergo every time. as another poster suggested, if my kiddo on my back was flailing around, i would stop to tend to that kiddo. if i had a choice between an ergo and a front seat, depending on the age of the kiddo, i would most likely choose the front seat. i have had many instances, even, of my kiddo falling asleep in the front seat, and was easily slipped right onto my back to be tied in the ergo once we got to our destination. while i have used bike trailers as well, they really don't even rate on my scale of practicality---while i know some people are all about them, they have always seemed dangerously unsafe to me (yes, in my experience of riding with them), not only are you super detached from whatever your child is experiencing on the ride, but even with flags, lights, and reflectors, they can be super blind spots when riding with traffic. i also think they are heavy and encumbersome.
just my opinion, based on my experience, which in this arena is pretty hefty. OP, i suggest you do what feels safest for your body and your kiddo. that factor is going to change for everyone on here. i have had a few times where i lost balance----only with the back seat. that's how awesome that fishtailing sensation is. if i were to have ever fallen with my little one on my back, i am pretty confident that i could land as a cushion to said kiddo. much softer then the impact of a plastic seat.