What variables should I consider when choosing between gear that is hot forged versus cold forged?
Hot and cold forging are different means to a similar end, like starting a fire with whiskey versus gasoline. A manufacturer will choose one process over the other for economic reasons, or because one process more efficiently renders the product.
Forging is simply the working of metal by plastic deformation, and it can be done while the metal is hot, cold, or even warm. If you watched the swordsmith hammer your samurai sword into shape, you have a grasp of what forging is all about, although in the case of climbing gear, it is a bit more complicated than that.
Do not confuse forging with machining, which is when you shape metal by removing material, usually by drilling, sawing or milling. Machining is a slow and expensive process reserved for items that must be precisely built, such as cam lobes and crankshafts.
Forging is also different from casting, where you pour molten material into a mold. Casting is cheap and can produce a complicated shape, one that would require numerous machining operations, in just one step. But, because casting randomly aligns the molecules, cast goods typically aren't as strong as forged ones. Iron and aluminum patio furniture, lawn jockeys and tin soldiers are often cast. Few, if any, pieces of climbing pro are cast, but the process is common for ascender cams and ice-axe heads.
Hot forging involves heating the metal, then shaping it. Usually, a machine press smashes the hot metal in a die, which might also be heated. Because the metal is hot, it is easy to move around, allowing for more elaborate shapes than cold forging. Hot forging is common for harder metals such as steel, that would be difficult to shape when cold. Some of the more sophisticated carabiners and most belay devices are hot forged. Depending on the metal and the degree to which it was heated, the forging process itself might suffice to temper, or strengthen, the material. Usually, the product is additionally heat treated after it is hot forged.
Cold forging is done when the metal, usually aluminum, which is soft and easy to work with, is cold. Cold forging is typically less expensive than hot forging, produces a precision product that requires little finish work, and is often used for symmetrical items such as carabiners. To cold forge a carabiner, aluminum bar stock is bent to shape, then smashed in a die to refine the shape. Because soft, non-heat-treated aluminum is weak, the carabiner is heat treated to strengthen it.
In a practical sense, it doesn't matter how a piece of gear is manufactured, excellent examples of hot- and cold-forged equipment abound. More important are price, strength and function. So just buy what you like. Got it?