We used them both of them for a while to varying degrees of success. How old is Liam? We used the sleep positioner (the little cushing things that you put on either side of the babie’s sides) for a while. I think I stopped maybe around 4 or 5 months or so. She started moving more and one day got stuck with her arms above her because she had moved down too far!
We also used the mattress wedge (raised the head of the mattress) until Dagney could roll over. Once she rolled over i was afraid her little face could get stuck in the angle where the wedge met the crib (I know it says to put it under the mattress in the directions, probably for this reason, but we didn’t think it would get crushed under the weight of the mattress). Now we just have books under the legs at the head of the crib.
I would say the little positioner that you put at their sides is worth it, but I think the books under the legs probably work just as well, or better because you can make it higher or lower as you prefer it.