Pretty simple: if you are betting on suspenders, forget about the belt. If you are going for a belt, don’t wear suspenders. Visually, it gets way too much wearing them together. Plus they are both there to hold your pants in place, so wearing both in the same look is kind of redundancy.
Having said that, your pants should never need a belt or suspenders to be on its place. If you need one of them for that, then your pants’ fit is a bit off.
And yes, you could wear none of them even if your pants have the belt’s loops. It is an option.