Actually both of them are really good in cryptocurrency and they have both advantages sometimes there are deferent but both if them are good. Coinmarketcap you can check almost all the details of each coin and tokens in coinmarketcap. In coingecko we can check the price at the same time we can also calculate all your token price that makes you easy to know if how much money you earn if you want to sell your tokens.
both are good, I use both to reference some altcoins that I got from the bounty campaign. because some are already registered with CMC and some are only registered at Coingecko. I also saw blockfolio, but now the calculations are not as accurate as before. sometimes there are price differences that occur in the actual market.