I think buying bitcoin using a credit card is not very attractive to me because it incurs a large transaction fee between 3% -6% depending on the credit card and the exchange party. So I prefer to buy bitcoin by using a bank transfer because the cost is not too large so I can get more profits.
All operations with credit cards, even when you deal with fiat, need commissions. Thus, using exchanges is much better. Cool if you have a list of trusted exchanges and monitor their exchange rates to pick the better one.