The welding neck flange is normally referred to as the "high hub" flange. It is desingned to transfer stresses to the pipe, thereby reducing high stress concentration at the base of the flange. The welding neck flanges is the best designed butt-welding flange of those currently available because of it's inherent stuctural value.
The Silp-on flanges has a low hub because the pipe slips into flange prior to welding. It is welded both inside and out to provide sufficient strenght and prevent leakage. Slip-on flanges are all bored slightly larger than the outside diameter of the matching pipe. They are preferred over welding neck flanges by many users due to their lower initial cost, but final installation cost is probably not much less than that of the welding neck flange because of the additional welding involved.
The socket welding flange is similar to a slip-on flanges except it has a bore and a counterbore dimension. The counterbore is slightly larger tha the outside diameter of the matching pipe, allowing the pipe to be inserted into the flange similar to a slip-on flanges. the diameter of the smaller bore is the same as the inside diameter of the matching pipe. A restriction is built into the pipe to rest on. This eliminates any restriction in flow when using a socket welding flanges.