A few side-by-side photos and some measurements for comparison. The measurements are approximate. I apologize for the blurry photos--photography is not my area of expertise!
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I was surprised that the Prime clone had the largest air flow diameter at 2mm. It feeds a 1mm x 3 mm slot directly above the opening. The SXK Doggy is 1.8 mm. The Galaxies RDTA is smallest at 1.5 mm but there are two additional air flow screws on the sides that can be opened up for those who desire more air flow to the coil.
The deck on the Galaxies is plated.
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The Galaxies is a RDTA so it does not have a separate chimney. The thickness of the top cap is the chimney. It has the shortest distance between the coil and the drip tip.
The Prime clone has the longest distance between the coil and the drip tip.
The chimney on the Doggy is plated.
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The bell on the Galaxies is the largest since it is also the top cap but it has raised area directly above the coil that forms a smaller secondary bell.
The Doggy has the largest chimney internal diameter at 5mm. The Galaxies RDTA and Prime clone both have a 4mm chimney internal diameter.
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The glass for the Galaxies is thicker than the glass for the Prime clone. It seems like there’s room for thicker glass on the Prime clone and it’s my guess that the German-made Prime might have thicker glass but I’ve never seen the authentic Prime. The Doggy has a threaded plastic tank.
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The airflow adjustment for the Galaxies RDTA is above the tank and built into the top cap. You have the options of opening a single hole or two holes on opposite sides. You can also slightly offset the alignment to open the holes part way.
The airflow adjustment for Doggy is a ring that slips over the base. There's a single slot of equal size on the ring and the deck. Offsetting the alignment closes off the opening part way.
The airflow adjuster on the Prime has six different size holes that are rotated and locked into place. The adjuster is a hexagon, not a circle, so there are no in between settings.
And here are some dealer photos that show everything that is included with each.
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