I've tried a front mounted and found it was very difficult to steer as all the weight is on the handlebar. Bumps, bounces and sudden swerves were nerve wracking. Every bump made me hang onto the handlebars tightly since all I could think was 'don't crash' since me crashing meant my daughter would go flying along with the bike. I also had to change the way I pedaled to accommodate the seat and child.
The rear-mounted seat was like having a counter balance weight on the back. I went one way and she went the other throwing me off completely.
The trailer was, and has been, my favorite option by far. The trailer is on an independent swivel so whatever both kids do back there doesn't affect me. Vice versa if I were to crash the kids are totally fine since they are not riding on my bike. Its also grown to allow for two kids and has been a blast with both of 'em.
I have a fully tricked out mountain bike since single track used to be a favorite pastime. Now I'm sharing the love of cycling with my kids and so far haven't managed to scare them too much.