Age
The Cloud and Wind are the two original ships built in 1994 and 1995. The Shadow and the Whisper are the two new ships built in 2000 and 2001.

Size
Cloud and Wind, 16800 tons 296 guests
Shadow and Whisper 28,258 tons 382 guests

Cabins
The Cloud standard cabins are 240 sq.ft.
The Whisper standard cabins are 287 sq ft.

Many people like the Cloud and the Wind particularly those who have sailed on them since the beginning. Kind of like those that loved the Song of Flower. Personally I highly prefer the Whisper or Shadow. I just find the newer ships are more comfortable, bigger cabins, higher ceilings.