A glass on glass stopper on a whiskey decanter can be a good choice for several reasons:

1. Aesthetics: A glass on glass stopper can add a touch of elegance and sophistication to a whiskey decanter, and can complement the overall design of the decanter.
2. Preservation: A glass on glass stopper creates a tight seal which can help to preserve the whiskey inside the decanter, by keeping out air and light that can cause the whiskey to spoil or lose its flavor over time.
3. Durability: Glass on glass stoppers are typically made of high-quality materials and are designed to be sturdy and durable, which can ensure that the stopper will last for a long time without breaking or cracking.
4. Easy to clean: Glass on glass stoppers are easy to clean and maintain, which can help to keep the decanter in good condition and prevent any buildup of residue or bacteria.

However, it's important to note that glass on glass stoppers may be fragile and can be prone to cracking or breaking if dropped, so they should be handled with care. Additionally, glass on glass stoppers may be relatively heavy, which can make them less convenient to use, especially if you need to pour the whiskey frequently.