We decided to buy the Burley Solo because it was safe, very light weight, as narrow as a bike is wide and because of the reputation that Burley has earned among cyclists, particularly friends of ours who own one.
We have only 1 child so this was the right choice; if we had another child we could buy a second Solo and have the stronger rider pull the older child plus lunch/gear while leaving the other rider to pull the lighter child. Since older children do not nap at the same rate as the younger ones, by having separate trailers we would be able to keep the dearly coveted nap cycle intact.
Safety 
the Solo has a protective roll cage that completely surrounds the child, from above, on the sides and below. There is a recessed helmet pocket which allows extra room for the child to move their head around with a helmet on. Burley has a five point harness system (seatbelts) that keeps your child snuggly in the seat while being comfortable so your child does not protest when you put it on. Burley has 3 choices for hitch, and each hitch has a secondary safety strap. We picked the most bombproof of the hitches, the Alternative hitch pictured to the left which secures the trailer in the most secure fashion, directly to the rear axle of your bike; the trade-off for the increased security comes with and increased initial set-up time of about 30 seconds, truly a no brainer deal. Be sure to specify when you order that you want the Alternative hitch.
Weight; with a low weight of just 17.5 pounds the Solo is the lightest child trailer on the market. The 4 pound savings over a regular Burley is easily noticed on any incline. Having rented one trailer that weighed over 30 pounds and the the extra weight was an issue each time we used it, we had made it a key point to get the lightest trailer we could.
Width; the Solo's narrow profile width of 28.3 inches. The narrow profile had many advantages we wanted to avoid having to dread pulling the trailer out for rides. The width of the Solo was equal to the width of my handlebars, I was confident that if I could fit through something so would the Solo - no small point as hitting the edge of something at 10-15 mph would be a devastating collision for our child: the Solo freed us from that kind of worry. Another plus for the narrow profile was our wind resistance would be cut down considerably which would make pulling the trailer easier and more enjoyable and thus the trailer would be used more often. The Solo is 4-5 inches shorter than other trailers, also contributing to even less wind drag.
Reputation; Burley's reputation in the Trailer world is legendary, something we discovered when we talked to other parents with bike trailers. The life-time warranty only backed up the reputation. Burley is unique in that they planned for these trailers to last and to be used; online they sell replacement parts for current and past models, if you break something you can easily replace. Other trailer brands did not offer such a feature. If the price of a new Burley trailer gives you pause, it is a sure bet you can find one on eBay for a less; knowing that you can replace parts makes buying a used one less risky. There is a healthy market for used Burleys, so selling your new one after your child outgrows is likely going to be pretty easy.
