Long term investment is not related to Spot, Margin or Derivatives.
When you invest long term, you must keep your coin in your wallet, not on exchange. If you mean Spot, Margin or Derivatives, you must store your coin at exchanges. That is bad in security for your coin and capital.
In addition, if you invest and store your coin at exchange, you can easily to go with trading because it is very convenience to trade on exchanges. Then you will change from long term investment to trading and it is bad.
Thanks for the advice but these are only hypothetical queries so when reading them please imagine, hypothetically, that wallets don't exist and exchanges are the only place to hold crypto.
It should be remembered that with margin trading, your position may be liquidated and you will completely lose your money. In addition, margin trading implies the use of borrowed funds of the exchange, and you will need to pay an hourly commission for its use. So a spot purchase will be safer for you, although it will bring less profit than using margin.