i use a liberty front tree
converting the front end to a tri glide front end is much cheaper than the liberty tree, but the liberty tree provides less steering effort. & since hd quit making sidecars the best option is a liberty sidecar - they are very good looking, have a softer suspension, more options [except they dont offer intercom and speakers but using hd parts this can be done] and excellent paint match. liberty was my second choice and after the fact id say that liberty would be my first choice over a used hd unit.
if the front end is changed to a small trail and the alignment is right they are actually a bit more stable than a trike and they have more room than a trike. the ride of the sidecar is just like a 50 caddy - smooth and floats. however if the rider is jerky with the throttle the sidecar body will rock forward and back quite a bit