謝謝回答，感覺每個人水平不一樣，沒有 margin 保護是個麻煩

Commonly, a trader who primarily deals with futures might opt for option 1.
This is mainly because this option allows the fund to be immediately
available as a margin collateral. This is critical in futures trading where
positions need to be rapidly entered and exited. The drawback of not earning
any interest on these funds is often outweighed by the advantages of having
the funds readily available for trading.

For an individual who trades both equity stocks and futures, option 2 or 3
might be a better fit. The ability to earn interest on the un-used fund and
the higher insurance cap, especially in option 3, becomes more attractive.
Whether they choose option 2 or option 3 would depend largely on how
frequently and rapidly they need to move large funds in and out of the
market.