AS2129 TABLE D FLANGE manufacturers tell you that flat welding flanges are generally used for low and medium-pressure pipelines, butt welding flanges are used for the connection of medium and high-pressure pipelines, and butt welding flanges are generally at least PN2.5MPa. AS2129 TABLE D FLANGE manufacturers tell you that Butt welding is used to reduce stress concentration. 

AS2129 TABLE D FLANGE manufacturers tell you that generally, butt welding flanges are mostly neck flanges, also called nipple flanges. Therefore, the installation cost, labor cost, and auxiliary material cost of the butt welding flange are higher, because there is one more process.
AS2129 TABLE D FLANGE manufacturers tell you that not all butt welding flanges need to be welded on both sides of the inner and outer sides, and generally, only one outer side is welded without special requirements. Flat welding is better for welding because the pipe and flange are perpendicular to each other, and the pipe will not be inclined.
The above information is the AS2129 TABLE D FLANGE manufacturer's introduction and analysis of the difference between flange, hope it helps you.