Summer/Winter: use whereas
The weather is hot in the summer, whereas in the winter it is cold.
Mice/Elephants: use on the other hand
Mice are small. Elephants, on the other hand, are big.
Trains/Cars: use whereas
Trains run on tracks, whereas cars drive on the road.
iPhone/Android: use on the other hand
Android phones are made by many different companies; on the other hand, iPhones are only made by Apple.
France/China: use whereas
Whereas China is located in Asia, France is located in Europe.
The beach/the mountains: use on the other hand
You can swim at the beach. On the other hand, you can hike in the mountains.
Pencil/Pen: use whereas
You can erase what you write with a pencil, whereas with a pen you cannot.
Vegetarians/Vegans: use on the other hand
Vegetarians can eat dairy products; on the other hand, vegans cannot.
