I own many of Atomic and Scubapro regulators. I have been using Scubapro since early 90's and Atomic since circa 1998. They are all top performers but Atomic has an edge in performance and in reliability. Most recent example, I just got an Mk19EVO with G260 I took with me on a trip to the Red Sea a couple of weeks ago along with an Atomic ST1. the MK19/G260 is high speed BMW where the Atomic is a solid Mercedes. I put the ST1 second stage on the MK19 with the G260 and alternated breathing from both underwater. The Atomic second stage is "smoother" breathing with its "automatic" venturi where the G260 is a wild horse with lots of power. I say that I prefer the Atomic second stage.
One other important factor, the Atomic regulators with sealed first stage and the seat saving feature in the second stage and all different metals in different parts of the regulator in the different models all give you a much higher degree of reliability and much, much longer time between servicing than any other regulator in existence. The Scubapro regulators are very reliable and very high performance but, in my taste, the Atomic has a definite edge there.
Anyone that tells you there is no difference or that you won't notice it, they don't know what they are talking about or they don't have much experience with the regulators (that's like saying all women smell, taste, feel and sound the same in the dark). FYI, I own a shameful large number of Atomic and Scubapro regulators and I am always comparing in the pool and in openwater. I can tell the difference in blind tests with my hands behind my back (when the regulator are tuned to mfg. specs).