I want to buy bitcoin with margin, but I am not sure which is the best way to do it considering commissions.
Practically there is 2 ways
To buy bitcoin futures or lending USDT and buying BTC
Which is the cheapest way to buy it?
Best of course, use a leverage. But if you do not have trading experience, I would first practice in your place to trade without margin. Since margin trading can incur big losses for you. I recommend the bitmex exchange.
You are giving him conflicting information. If you said that margin trading is risky and he shouldn't use margin but yet you told him to use Bitmex anyways, a derivatives exchange with 100x leverage.
If you don't want to use any margin or leverage at all just use Coinbase, Gemini, Kraken and buy on the spot market with your USD and leave it as is. If you want to use leverage then you can use any of the mention exchanges however right now its generally not a good time to buy BTC or any other alts on margin because the lending and funding fees or future premiums are high because too many people are bullish.
Either wait for it to cool off and the margin rates to go down or just buy it on the spot market with no leverage and you won't have this issue. You just those a bad time to go long, its a great time to sell or go short right now.