Pillow top matress?

We have a little one due very soon and DS is still going strong in his crib so we are looking to buy another crib matress. DH wants to buy DS (who is 19 months old) a softer matress and give the new baby his more firm matress. He really wants to buy him a pillow top matress (cause he thinks he would love it) but I'm just not sure about it.

Would it cause any harm to his spine or anything like that? I have no problem buying him a softer one as long as its not in any way harmful to him. I know their little spines aren't fully fused yet....

If anyone has a link to a softer mattress they recommend, please share!  

Thanks!!