They’re in different mountain ranges, and it shows in everything.
Yercaud is in the Eastern Ghats at 1,515 metres; Ooty and Kodaikanal are Western Ghats at 2,240 and 2,130. Yercaud is drier, its forest is shola and silver oak rather than rainforest, and its estates grow coffee, pepper and oranges together rather than tea.
The practical differences matter more. Yercaum is one hour from Salem Junction, where Ooty is five hours from Coimbatore. There’s no permit-controlled road, no toy train to book sixty days ahead, and a fraction of the traffic. It’s smaller — you will not find Ooty’s range of hotels or restaurants.
If you want a famous hill station with plenty of choice, take Ooty. If you want the one Ooty was fifty years ago, come here.