I have used a trailer, trial-a-bike, and rear seat, but not a font one. My concern with the front was restricted pedaling/steering, and in case of quick dismount I would not be able to move forward.
Of the others, I have used the trail-a-bike and trailer with a full susser. The trail-a-bike attaches to seat tube, but is only suitable for older kids (probably four upwards). But it is good, gets them pedalling as well and is highly enjoyed by my kids.
The trailer attaches to the chainstays, so again is fine with a full susser, but not rear disk brakes. It is great for two small kids, and coats/picnicsetc.. but with the kids so low down I'd be wary in any traffic.
Rear monted seats (either on a rack, or seat post) are OK, but they alter the stability of a bike quite markedly, and cycling on your own there it is awkward getting the kids in or out.
I'd be wary of using all of these on busy roads. The trail-a-bike is the only one I do use on road, and even then it's to take the kids to school or potter around locally on quiet roads.
One last comment - trail a bike and or trailer are both more expensive, but you'd recoup some of that when you sell on.
Hope that helps