I don't see any reason why the C channel wouldn't make a rigid frame. Are you planning to TIG weld the corners or bolt them together with right angle pieces?
Belts have some minor issues with tension and routing path, and I suppose that two steppers can get out of sequence if there is binding on one side versus the other. If one side starts missing steps it will cause the gantry to get out of alignment and bind. I have a dual leadscrew with belt (Solsylva) and may try the dual stepper drive in the future if the belt presents too much effort to keep it running right. I haven't seen any real complaints about belts so far, just mostly questions as to which way to go with new builds.
For that size machine an approximately 270 oz/in should be fine. You can find the formulas to calculate it on this site or just go with what equivalent size machine builds have used.
Maybe they were too jealous of your C channel pieces to comment.