trailer type aside, an important factor to take into account given the bikes that you both have, is gearing.
The gearing on your bikes are going to have a low gear of X gear inches--most likely your bikes have 50/34 cranks and maybe a 28 or 30t at the rear, so with a 34crank and 30 rear, with 28mm tires, this comes out to 30 gear inches low gear.
With either trailer, with a given amount of weight, you will quickly find that on hills your gearing is way too high for enjoyable riding (and your knees).

Unfortunately if you havent ever ridden a bike with load on it, you may not have a reference, but depending on where you ride, your bike as is with trailer may be a bit overgeared, quite a bit overgeared, or really really really overgeared with a given load on a given trailer.

The only thing I can suggest is to try to borrow a bike with panniers and put X lbs of stuff in them (about what you expect to take on a trip with your son) and see how the equivelant low gear of your road bike is going up some hills.
There won't include the weight of the trailer itself (obvoiusly a bit more), but at least it could give you an idea of what X gearing feels like on X hills with X weight.