I follow two rules: Little funds to exchanger  - big funds to wallet. If a wallet is secure depends on the fact, if you own your private keys. A third party wallet is as unsecure as exchangers.
Here you can find great infos about wallets:
https://bt.irlbtc.com/view/1631151.0

The people who do an exchange wallet storing is fool and they are putting their 1 foot on grave since Exchangers are not really safe since there would be a breach on the system which could lead them up to hacking. And its so advisable by the experts to store our balance on ledger or hardware wallet since this one will be the safest place where we can store our money without worrying of anything.

I suggest keep your coins on wallet atleast you will hodl your own private keys but you must write it down and keep it secure if incase you will lost it you have a back up because that is the only way to access your wallet.

i use both,for some tokens i use wallet to keep it, but for real coins i don't have any hardware wallet so i use exchanger and 2fa. i think it is already safe if i not do any strange thing like click on suspicious link

If your a day trader, better to keep part of your portfolio for trading in the exchange. This way you will minimize your transaction fees compare to a hodler. But if your a hodler always keep your funds in the wallet which is more secured because you have the private keys.

If you are planning to trade your coins you should put it on exchanges because you cant trade using your hardwallet,but if you are going to hold your coins for a long time i suggest put it in a hardwallet because these wallets are most secured storage today,ledger or trezor wallet are the most common wallets that are being recommended by the experts.Exchanges doesnt allow the users to store big amounts of coins if they are not trading because exchanges is more prone to these hackers nowadays.

I don't have a definite answer to your question. because sometimes I keep some of my altcoin in exchange. this is because I am a trader, and if the altcoin I buy and I hold for a period of time, then I will put it in my wallet. yes, I guess everyone is doing the same thing as what I do.

If we reworded your question to which is better: cold storage wallet or exchange the answer is cold storage wallet.. but if you're talking free, everyday wallet with new users and very low security.. I'd probably trust the exchange more.  To me its alot like plane crashes with exchanges.. its very very VERY rare that an exchange would get hacked, but if they do alot of people are going to get hurt.. but your wallet like a car is less likely to have as many casualties..but you personally with very little gard for personal precaution are much more at risk.

I think its not that bad to hold in exchange wallets if you set nice password and 2fa. You just need to not using suspicious unknown exchanges and everything will be okay, holding in personal wallets can be quite harder than this.