Kochi is better as rainfall in Kerala is generally pours. This is assuming you actually want enjoy rains, in spite of the associated discomfort. Monsoon in Kerala is something a nature lover will thoroughly enjoy. Later part of June onwards is when the rainfall is heavy.
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Both Kochi and Yercaud will have their own charm in the rains.
Yercaud being a hill station will naturally be misty with lots of streams and waterfalls, hence perhaps more attractive. Check for landslides during heavy rains.
