It's better to store your bitcoin in your bitcoin wallet more safe and more trusted than investing but we know that now all cryptocurrency exchanger are trusted there are also exchanger that scam money when you do exchange.
Well probably choose the most reputable wallet in your country. For example here in the Philippines, I found it convenient to store my Bitcoins in Coins.ph. Or, you can make your own hardware wallet, or a paper wallet at least. I highly suggest not to put it in exchanger sites, you cannot guarantee that will not run in a long period of time.
Online wallets are not safe either, even if you trust coins.ph, it is better not to store large amount of bitcoin in there. And you do not hold the private keys in coins.ph, so you actually do not have control over your bitcoins if they decided to become scam. Since you are investing for long term, hardware wallet will be the best option.