You can focus on a shoulder rig and you can't when flying, well at least not without an electronic follow focus and another person to pull/operate it. Video cams with a deep DOF on infinite wide focus work on many cameras while others use auto focus while some operators try to stay within a specific focal range.
From personal experience, shoulder rigs are great, but the temptation is to overuse it and forgo creative use of your tripod.
I have the Glidecam X-10 system and don't use it nearly as much as I thought I would. You'll get more use out of the shoulder rig IMO.