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Square tube (or I-beam) is a trifle stronger than C-channel/C-beam. You may not need that additional strength, depending on exactly what you're building. C-channel is a bit lighter, which may not matter for your application. (I'm using square-tube aluminum for the bike trailer I'm modifying. C-channel would probably have been fine for this application, but ...
The major advantage of c-channel is that you can actually paint all of the surfaces so rust is manageable. With tubular, you can't protect the inside from rust and corrosion. I would use c-channel for that reason alone.
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