Both are useful tools but CMC is more popular than Coingecko! Coingecko has something unique features like Exchange trust score, ICO review and many more but CMC only has token price chart, trading view and exchange rankings which also have in Coingecko! But CMC has some strong rules to get listed there where any coin/token project easily can get listed in Coingecko, that's why it has more coins listed than CMC! To me, this lot of token listing is a drawback in Coingecko!
You are right. CMC and Coingecko are very useful and have their respective advantages that stand out. Where we know indeed to be registered in CMC, must meet the terms and conditions set, you can see it in detail
here. CMC shows the price movements accurately, precisely, and completely from all over the world, we can also compare the exchange rates of various exchanges of a coin or token to determine trading strategies. Because of its consistency and popularity as well as its accuracy, many make CMC an indicator of the success of a project if it is already registered there.