I've seen that these two smart contracts are often used, and the ETH is widely used more often than WAVES. What is the difference between the two platforms?
Waves is not a smart contract plattform - it is a plattform to issue tokens on a very, very simple way (tokens dont have any functionality).
Waves also has a so called DEX (decentralised exchange), where you can trade the tokens of the waves plattform.
Waves recently were doing pretty well, but people still remember, how easy it was to make a Waves token and most ICO's on Waves been scam. They need to do something about it and DEX exchange has not that great kind of volumes, even BTC/ETH and that is a strong weak point of Waves.