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!
I agree because way back 2017, almost everyday there is a new coin listed on CMC including shitcoins which I think the reason why CMC raise its standard for listing new coins. Coingecko is not new but competing with CMC's popularity is one of the possible reason why they accept new coin easily and one more thing is they want to get more traffic on their site due to the people searching for the new coin.