From what ive read
Water is actually a far superior medium in comparison to air for transfering temperature, a good example is placing a warm beer in the fridge and a warm beer in a bucket of water with ice...
Needless to say the beer in the ice water chills dastically quicker than the one in the fridge..
The problem is ambient air is usually below 40 degrees consistantly, no cooling required however the water temp in a water to air system can get above this if your system does not have enough water or the radiator to cool the water is under sized.
Thats why Air to air coolers are generally so much larger as they require a greater surface area to transfer the heat.
up side is usually less piping so less lag
down side is that they are more complicated to set up, they need there own water supply, pump and radiator.
Sitting at traffic lights without a thermo fan a water to air system will probably suffer greater heat soak than an equiveliant air to air system
Hope this helps