Carboy Cap vs. bung

Not sure what you mean by an orange carboy cap. If the type that have two holes (one for an airlock, and the other for an overflow hose), they work fine for their purpose. But after fermentation, there is excessive air-space, and the second hole has no function, and must be plugged. If wine gets into the "stems", it is an invitation to bacterial growth. Also, many of us prefer a solid bung after fermentation, rather than a drilled bung with airlock. Another factor is the price difference. #7 drilled rubber bungs are about 50 cents. The orange caps are several dollars. Not important with one or two carboys, but I have over 100 carboys aging now.

As to why the literature mentions bungs and not the other option: bungs have been around a lot longer.

Rubber stoppers provide a more airtight seal, but I've used caps on carboys many times without problems. One nice thing is that you can top _completely_ full and not worry about blowing the bottom out of a carboy. You do need to be vigilant for signs of oxidation however.

Tom S