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.