While antifreeze raises the boiling point of coolant, it does not dissipate heat as well as water. 40/60 coolant/water in theory is COOLER than 50/50. 60/40 coolant/water is HOTTER than 50/50. Some say that the ultimate "cool" trick is 15% antifreeze, water & 2 bottles of WATER WETTER(made by Redline). I've never experimented with this because coolant lubricates the water pump.
For what it's worth, I live in a fairly humid area of south Texas and I've experimented with 30/70 - 40/60 and 50/50. My MBs temp gauge reads the same with each of these concentrations. In the end, I suspect the best deal for your car is 50/50. Regards.