I have that exact front seat, and a fancier back seat (Rhode Gear Limo). I prefer the front one. I can peddle fine, although my knees go out a bit more than normal (which is better, frankly). The real advantage of the front one is, you can talk to your child, and they don't lose shoes. Oh, and they can sleep on the blue pad, rather than by resting their helmet on your butt.
I had a crash, once, with a child on the front and a child on the back. Neither had a mark on them. It was a pretty minor crash (only the front wheel hit the car, we went down fairly slowly), but still - neither child was hurt, at all. I had a few bruises, though.
The half-bike tandem thing is fine, but only when the child is old enough to hold on and not let go.