I have both, but because of the type of work I do, I use the table saw sled 90% of the time. I have several sleds. One is rather large for working on furniture components. I have a small one that's set up for making small parts, for banding and other delicate items.
I found this description. I did not write it, but it is accurate from my perspective.
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Here is how they compare in terms of practical benefits:
While a miter gauge is more compact, a sled offers several mechanical advantages that a gauge simply can’t match:
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Superior Workpiece Support: On a sled, the wood sits on a moving floor rather than sliding across the cast iron table. This eliminates friction between the wood and the table, preventing the workpiece from "drifting" or pivoting during the cut.
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Zero-Clearance Support: Because the saw blade cuts a permanent slot through the sled’s base and fence, the wood is supported right up to the edge of the teeth. This virtually eliminates blowout (tear-out) on the bottom and back of your cut.
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Safety with Small Pieces: Sleds make it much easier to use hold-down clamps or blocks. You can safely cut a 2-inch scrap without your fingers ever getting near the blade.
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Repeatability: Most sleds are used with a "stop block" on the back fence, allowing you to cut ten identical chair legs to the exact same millimeter every single time.
A premium miter gauge (like those from Incra or JessEm) is often better in these specific scenarios:
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Complex Angles: Adjusting a miter gauge to $22.5^\circ$ or $45^\circ$ takes seconds. Unless you build a dedicated "angle sled," a miter gauge is far more versatile for non-square cuts.
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Space Savings: Sleds are bulky and can be a pain to store. A miter gauge can hang on a wall hook.
- Long Miters: If you are cutting a miter on the end of an 8-foot board, a miter gauge with a long fence extension is often more practical than trying to balance a massive board on a sliding sled.