They both achieve the same thing different ways. From the shooter's standpoint I don't believe there's a practical difference. Since the gap can't be seen with the cylinder closed, there's not an aesthetic difference either.
If you're worried that the gap introduces some weakness into the design, don't. The SP101 design holds up very well to the .357Magnum cartridge. The cylinder lockup is quite strong and although there's a gap, there's steel-to-steel contact insuring proper alignment.
My guess is that Ruger's method is cheaper and simpler to manufacture.