I've heard both manufacturers' amps side by side, and I happen to like both of them a lot. With the exception of the Stealth, Ray Samuels' amps are SS, while the SinglePower amps are tube. That in and of itself is a pretty big difference.
Have you decided on a price range? If so, have you decided on a preference for tubes or solid state? This is where I'd start.
Both manufacturers' amps do a very nice job with bass response. If you like bass, you will not go wrong with either. Like most solid state amps, the Ray Samuels amps are forward in their presentation. They are, however, some of the nicest SS amps I've heard, with richness and detail that anyone who listens to tubes frequently will appreciate.
I must admit that I have a bit of a soft spot for the tube sound. SinglePower has several different tubed offerings, and represents a very good value IMHO. They are some of the sweetest, tuby-est amps I've heard.
I don't think that, with the exception of the Stealth vs Supra, the amps compare well with each other. They are very different animals. Think about whether you prefer tubes or solid state, and make your decision based on that. These two lines offer what I think are great complements to the other...why not just get two??