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balloon arch is usually a structure with two legs standing on the floor. It creates a clear arch shape and is often used for entrances, backdrops, or focal points.
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balloon garland is more flexible. Sometimes it may look like half or three-quarters of an arch, but it does not have to. It can be shaped in a more flowing, curved, or organic way. It may have one main support point rather than two legs, and it often looks softer and more interesting for the eye.
For baby showers, the right choice depends on the room, ceiling height, available space, and the feeling you want to create.