is it the foam or the coils that makes you sink into a mattress?

I bought a new simmons beautyrest with pillowtop and when I sit on the bed, I sink deep into it. When I move on the bed, I roll back into the pit I created where I was. I’m going to switch for another mattress, but I’m wondering what is causing this. Is it all the layers of foam (memory foam does this I think) or is it the individual spring coils? My upper body sinks in and my feet (that weigh less) are sort of up in the air, giving the feeling of sleeping downhill.

Should I get a traditional continuous spring mattress (maybe a stearns and foster) with minimal cushion on top? Or just another simmons beautyrest but without the pillowtop (a “firm” or “extra firm” instead)? Hubby and I like the idea of non-transference of movement (I move a ton and he doesn’t move an inch) but if it’s going to make me feel like I’m sleeping “downhill”, then we’ll trade that in for firmness.