I probably would not get either of them. I would especially avoid the Sherwood SR2, if it's anything like the SR1. That reg is nothing at all like the old bulletproof sherwoods that made the brand's reputation. Its an over-engineered poster boy for what has gone wrong IMO with regulator design in the last several years.
The atomic regs are based on the scubapro balanced piston 1st/barrel poppet 2nd stage design with some refinements, but IMO they're not really any better. I would be happy owning atomic regs but I can't see paying $500 for a set, especially used. There are loads of MK20/G250 sets out there for half that price that are every bit as good for actual diving, and very easy to get serviced.
I will admit that I have a preference for older, classic reg designs that are simple, robust, and have been proven on literally millions of dives. I'm pretty sure regulator companies keep coming up with new models and "features" so that they have stuff to sell repeat customers, when truthfully the older models are fine for a lifetime of service. Unfortunately with the limited market of scuba diving, selling a diver one regulator for his entire dive life is not much of a profitable business model.
There are other perennial best buys out there, like the aqualung titan, older sherwoods, and earlier model scubapros. You could find a MK2/R190 for less than $100, get it serviced once, and use it for several seasons worry-free.
