I think it’s safe to say that once one has tried the current Tiffen (Steadicam) arms, it’s hard to go back to anything else. While all of the competitors and the older lower-end Steadicam arms are quite springy and require significant force to hold in their upper and lower positions, the newer models (Merlin/Pilot, Flyer, G50 and G70) are effortless in comparison. Aside from the comfort afforded, the isolation from the operator’s footsteps is improved as well. This all adds up to a greater useable boom range, less fatigue and smoother shots.
This is not to say that it is impossible to get decent shots from the other manufacturer’s products (although a few of the cheaper brands challenge this assertion), but it is harder, and thus requires more attention to overcome the design deficiencies.
As far as dual vs single arm models: in general it can be very difficult to get a single arm model to boom up high enough to get the camera to eye-level. If the subject is taller than the operator, it will likely be impossible. Having a 2-3 foot boom range is one of the great assets of Steadicam, and single-arm models cannot achieve this.
Outside of the arm, the other most relevant component to consider is the gimbal, as mentioned previously. The more friction and less linearity a gimbal the demonstrates, the more it will fail to isolate the operator from the rig and the choppier the footage.
These are the two moving parts of a rig; everything else becomes secondary once a shot is in motion (assuming that the rig is rigid enough to not present slop or vibration). However the type of adjustments available will greatly affect one’s efficiency with balancing the rig, which becomes a time factor for setups.