I haven't got that far yet but I think it's mostly to do with the air flow and posture. I doubt the foam ones would last quite so long as the sprung ones.
Mothercare have a good guide, even if they do only include their matresses!
The sprung ones are more expensive and feel comfier (I had a little lie on them lol). We have both a foam one (for staying at my mums house) and a sprung one for here, which cost around £60 and is fine. Id say to go for sprung and get a lovely quality one, it's def worth it.
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