Trailers are good and allow them a bit of freedom of movement as they get older (and offer toy and picnic storage) seats give them a sense of speed and if you get a cross bar mounted one they let them see what it is like to ride a bike, rearmounted seats can make the bike a bit unstable and you risk kicking them in the face as you mount up.
I wouldn't put a kid under 18 months on a seat though or a kid in a trailer under 12months, no matter what the recommendations are. They have to be able to properly support their own head weight, especially whilst being bumped about.
Someone on here is selling a Chariot Cougar2 - expensive but looks great and you can get skis for it which has to be a bonus .