I am not comfortable with the idea of a bike seat behind me because I'd be worried about the weight and me being able to keep my balance. I consider myself a weakling.
So I really prefer my Burley trailer.
One argument I read online against the bike seat behind the rider is that all the bumps on the road are felt up the spine of the baby/toddler. Another thing is... if I happen to fall over, the Burley won't. It will stay upright, so kids are safer.