First of all, let me say that the thickness of the 2 1/2 inches tube rose a doubt in my mind: what material is the tube made of? If it's stainless steel, forget about heating it up. You'll have to bend it with a manual tube bending machine, those hand operated machines that have a hydraulic cylinder.
If the tube is carbon or low alloy steel, then without any doubt oxy-acetylene is better. With propane you'll waste a lot of time until you get up to the desired temperature. This is so because among all hydrocarbons, acetylene is the one that has the highest heat of combustion, i.e., developes the highest temperature when burning.
A common torch, i.e., those used for welding, are better.
Giovanni S. Crisi
Sao Paulo - Brazil

If you take fire bricks (or common bricks if you don't have much to do) and build a partial enclosure to heat the pipe in then the hot bricks will reflect a lot of the heat that is being lost now back onto the work. Then your weed burner should work much better.
Bill