Sweep. One needs to set up the profile correctly with regards orientation, and placing the profile at the correct position at the start of the part. may take longer to set up, but it is more predictable than rail sweep.
Rail Sweep. TC automatically rotates and moves the profile to the start of the path and carries out the sweep, so the profile and path can be on the same plane. Manually Orientating the profile is not required, but results are not always easily predictable (it generally orientates Z axes to follow the path).
I tend to use Sweep the most, and orientate the profile how I want before carrying out the sweep. I just use rail sweep for a circle along a spline path, as the orientation of a circle just needs to be perpendicular to the start point, and rail sweep will do that just fine.