At one point I had both Mk.III 105 and C+ 105 and the C+ is it's own animal. The Mk.III is smoother and much more modern sounding with a compressed feel when heads up to the C+. The C+ is like a chainsaw or dirt-bike, where as the Mk.III is like a race car.
With the right guitar, you could get either to sound very similar because a guitar will change everything.