water: heat transfer capability means more heat can be absorbed by the water away from the engine but water has a less boiling point (100 deg C) compared to coolant. so if na-reach na ng tubig ung boiling point nya especially in hot and long driving, there's a lot of steam generation trying to escape out of the radiator (overheat)..

coolant naman, although less ang heat transfer capability nya, they have high boiling point naman (130 deg C). may anti-rust and lubricating property pa..