What do you want to do on the rig? The two configurations have some significant differences.
Trikes are fairly expensive, sidecars are too. They both possess "interesting" handling characteristics. One of the characteristics (under hard braking) is called "ambush", so be aware there is some training required.
I have seen a "bolt-on" outfit that converts a motorcycle to a four-wheeled "trike", but most other conversion methods are a serious undertaking.
Sidecars can be rigged to most any bike; some efforts are more heroic than others.
I've seen some rigs that are so extravagant, I wonder why they didn't just save their money and buy a convertible.
I had never broken any bones in my body after 25 years of constat motorcycling. Then I fitted a sidecar. Try and find one that comes with free underpants - you're going to need plenty. They slew right when braking (in Australia) and drag left when accelerating. The steering is VERY HEAVY and needs to be managed differently as the speed varies (due to wind load on the sidecar at high speed). Oh yes! I nearly forgot, They steer oppositely to a solo bike!!! But don't let me put you off.