we use a trailer all the time. the guideline we used for age was 18 months, as not only does the child need to be able to control their own head movement, but they also need to support a helmet. my wife wanted to get a bike seat and I was against it, using the before mentioned argument that if the bike falls so does the child when in a seat. another advantage to a trailer is that if the child naps on a longer ride they will be more comfortable. they're also perfect for the child to have water and snacks within arms reach. oh, and you always have to remember their favorite "blankie" or toy. none of these things can happen with a ride along seat.
if you do happen to flip a trailer (more details on how you did that loky1179, please), there is usually more than enough extra headroom to protect the child, assuming you were smart enough to strap them in.
though we've never needed it, our trailer has the seating space for two children of smaller size and the straps to allow that. your children are close enough in age that in about a year, this may be a need for you.
trailers are also good for training as you are basically pulling a 40 lb parachute. throw in one of those rides a week and you'll be surprised how much faster you are when you're without the load.
when your youngest is old enough, get a trailer and pack a picnic. we've had some great rides where all we do is go 20 miles to a park, eat some sandwiches and let the kids play at the playground while we rest up for the ride home.