Sheet metal vs tube is a resource decision. If you don't have the sheet metal bending capability, you are limited to COTS and tubing/channel/angle to get enough stiffness. If you do, it boils down to the usual COTS-vs-homemade decision - are we willing to take the time to design and build a custom sheet metal frame out of our build season to get exactly what we want, or do we take some COTS (extrusion) shortcuts to get something that's "close enough"?
Chain vs belt is a bit different. Among the pros of each are:
Chain:
- You can make any length you want (multiple of chain pitch, or better, 2x chain pitch), including much longer lengths than belts are generally available.
- It is narrower for a given amount of load; uses up less space.
Belt:
- It does not stretch over time like chain; works better with exact center-center axle spacing and no tensioners.
- It is lighter than chain.
- It stretches under shock load and is likely to stay in sheaves even when the belt jumps teeth. This allows it to be used as a torque limiter to prevent broken teeth and smoking motors, at a cost in belt life.