What can a diode laser do that a CO2 can’t — and vice versa?

It is a wavelength question. A blue diode (~455nm) handles wood, veg-tan leather, cork, slate, dark acrylic, and coated or anodized metal. A CO2 (10,600nm) does all the organics plus the two things a diode physically cannot: cutting clear acrylic and engraving glass.

Neither touches bare metal — stainless and brass reflect both. That is fiber/IR territory at 1064nm. The practical split: diode for entry cost and footprint, CO2 for acrylic work and production cutting speed.