I think the frames are a bit different and the C handles a bit more nimbly than the G because of the steering geometry and the 18" rims vs the 19" rims on the G.
That makes the G5 more stable when riding at freeway speeds.
I haven't run my G5 at 65 mph yet but from everything I've read both the C and G are getting into some heavy vibrations at that speed. (These bikes take about 1000 miles of break in before they should be ridden above 60 mph).
The C comes with a 18 tooth secondary drive (main chain to wheel) sprocket to spin the smaller 18 inch wheels a bit faster to make up for their smaller size while the G comes with a 17 tooth secondary drive sprocket to work with its 19" wheels.
I mention this because the 18 tooth sprocket is about at the max size for the housing on the engine.
That means you could change the G5's sprocket to a 18 tooth size to keep the engines speed down a bit at 60-65 mph. This
might help to reduce the vibrations of the G5 at those speeds.
The C5's use of a maximum diameter secondary sprocket means the only way to change the engine speed at 60-65 would be to change the rear wheel to one from a G5.
Of course all of the above is just my opinion and its worth every cent of what it cost.