I have lots of experience with the Addams Trail-a-Bike and lots more with the Burley Piccolo. Both on a single. The New Trail-a-Bike offers a decent ride, but the hitch develops play over time, so it's not good in the long run. I scrapped mine after 1800 km (I should have done so at around 1200 - 1400 km, but it took 400 extra km for the Piccolo to arrive. The Piccolo is very stable (up to 70 km/h) and the hitch doesn't develop play over time. Last Summer, Ève and I did a 11-day 750-km tour in Québec that included some decent hills too. Even though the bike plus gear weighted 150 lb, we had no problems riding it.
BTW, at 4 and 5, Ève did not contribute enough, but at 5.5 and 6 she contributed her fair share and at 7 she contributed a bit more. IOW, last Summer, there were definitely quite a few times that we were faster than I would have been if I had toured solo.
If I remember correctly, the gearing of the Piccolo is 35 to 75 gear-inches, so it's not great for steep hills or fast runs with the wind at your back. The other aspect is that, even with lower gears (mine is re-geared with 27 to 85 gear-inches approx.) it's easier to climb very steep hills with the child walking that with both riders trying to co-ordinate their efforts.
I have used the Piccolo a few times so far with the tandem, and once I got used to it, it rides well, even in the wind. For a more thorough outlook, ask me in a few months, however...
Finally, other aspects to consider if you use a Trail-a-Bike:
- The Addams doesn't like tight turns, unless you have lots of space.
- You need some seatpost exposed (2", I think) to attach the Trailercycle. Could be a problem if you have a child stoking.
- The lower the bike, the less clearance you have for a rear rack. With my 25" frame, I could even pack the tent on top of the rack; at the other extreme, I have heard of women whose frame was so small the Trail-a-Bike didn't clear a rear rack (i.e. no pannier-carrying capacity).