It's not exactly that one is "better" than the other.

If you take a plain old JRSC and install it on an LS, you won't get much of a gain. Maybe 30-40 hp tops. If you put a JRSC on a GSR you will get much more of a gain and have an insanely linear torque curve. So, people will say the GSR is "better" than the LS when you slap on a JRSC.

Now if you take the same turbo kit and bolt it onto an LS and a GSR, the GSR will still make more power. The tradeoff is that since the GSR has a pretty high compression ratio, you're more likely to blow up the motor than with an LS. That and you have more of a safety margin with the LS's lower compression. Still, a GSR running the same boost level as an LS can typically make 40 to 60 more hp at the wheels.

Basically with an LS the JRSC doesn't provide enough of a gain to really make it worth it, while a turbo does. The GSR will always make more power though.