Parts are harder to find for the RB20 in the US compared to the SR, there are alot more people familiar with the SR over the Rb20 in the US, while the RB is more efficient, it's about 50 pounds more then the SR and is alot longer and taller, dollar for dollar the SR is going to cost less for the same amount of mods. It's moot point though, as both motors are illegal for anything other then off highway vehicles.