Both are good cryptoexchanges but binance is already so established since it's already been doing the business for quite some time now. With so many cryptos listed on binance, i believe that binance is more utilized than its newly counterpart in the name of Kucoin.
Of course, Binance is better than Kucoin, at Binance we can see a very high volume of coins than in Kucoin,
and that I think this makes people prefer trading in Binance than in kucoin.
As you say both are good and I use them both. I`m active on binance since day one, but from recently I spend more time on kucoin then on binance or other exchanges, they have some good promotions, prizes, they send me mails all the time, they are very active on twitter too, there they have so many airdrops, and other things, interesting and I get something from time to time. Binance is great too, and I will not stop using them, so why to not use them both.