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.
There is no doubt Binance is the better exchange of these two, indeed the volume is much higher than in Kucoin but also the customer support of Binance is of high quality, they will almost immediately help you out and do it in a very proffessional manner, also on Kucoin 80% of the time your coins are not going trough, I had enough situations where I needed to contact Kucoin for letting my coins go trough, they allways have some kind of maintenance so my tokens get stuck there.