You might want to check out this thread http://bike-pgh.org/bbpress/topic/tips-for-biking-with-a-1-year-old#post-43098
and if you search the forums for "trailer" there are a few other shorter threads.
I believe the recommendation for trailers in general is for the child to be 1 year old before they can ride.
I've got an encore that I've been very happy with and is still going strong after 10 years.
I think you'll be very happy with a Burley purchase , but don't think you HAVE TO HAVE a Burley. Consider how much you think you're actually going to be using it and where you are going to be riding it. Will you eventually be hauling a 3 and a 5 year old? Look for the same kind of things you'd consider for a car seat or a bike for yourself and avoid department store models. I would start at REI if I were going to look for new one again, and lean toward the brands they sell, including Burley, looking for a used one.