I am a CMC (CoinMarketCapitalization) user because I know that years back there are some issue in regards to this site but now they are clean and i think best to try them both and you decide which is more functional and more trustworthy .
What was the issue with coinmarketcap? The only one I know, that it is being a bit laggy during huge price drops in crypto and panic on the market.
I am a coingecko user, as for me it loads faster and shows price changes hourly. But I suggest to use both, because one might have altcoins that other dont have.
The only disadvantage coingecko has is showing prices, when altcoin is not traded anywhere, and on on "Market" section there is only a promotion of eToroX.