[EricH]

Lighter or heavier depends on what you're comparing it to. A side rail like squirrel is describing is a plate, 3" high* 1/8" thick* 36" long. A typical drivetrain side using tubing has about the same weight, maybe a little more. Same length, but instead of 3" high, it's typically 1" to 1.5 " high, 1" wide, and 1/8" wall. So it will weigh a little bit more, as you effectively have to add an extra inch or two to the height of the 3" plate. Channel is lighter than tubing, because one of the sides is removed. This has other tradeoffs, though, because you lose all the support in the missing side along with the weight.

You'll end up about the same weight either way, due to holes and such like. If you really, really need the weight, a plate is easier to take material out of, but it's harder to get it removed from the right places.

edit: squirrel is right, for a channel that size. Most teams that use channel or tubing use smaller sizes or different materials.