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?
The cheapest way to buy it on leverage would be by a futures/derivatives exchange such as Bitmex. Currently their fees are only 0.075% if you are a taker or they pay you 0.025% if you use a limit order and provide liquidity. So basically you are getting paid to buy it as long as your order fills.
Now there are 2 types of contracts. One is called a perpetual contract and it has funding rates every 8 hours and the other is a futures contract.
The Perpetual contract is currently priced slightly above the spot rate due to huge demand for BTC however only by $10 or so. While the futures contract for March 2020 has a $300 premium and June 2020 contract has a $550 premium. Why is the premium so high? Due to high demand mostly. But with the futures you don't pay funding fees every 8 hours which currently only favor shorts.
You can use up to 100x leverage which I wouldn't recommend, use maybe max 5x or so. Anything higher and you can easily get liquidated.