What are the differences between the ECHO SRM-2100 and SRM-2601?? Other than bigger feul tank, and engine and weight? How can ECHO make an engine bigger by 4.2cc and only increase the weight by 1/10 of a pound?? Is the engine better design? Is it more "commercial"?Or did they sacrifice quality to acieve less weight and it isn't as good of an engine? Is the power that much more noticeable? What's with the rigid steel drive shaft? Is it better than cable and solid steel? What all do you know about them?