Cot bed and crib mattresses

Claire • First baby due in September.
Hi All.  A friend is giving us a crib and we have bought a second hand cot bed so we need to buy mattresses. I have looked about a bit but it's a minefield! There is so much choice and I don't know where to start. Any suggestions on where to buy and what to look out for? For instance, are the really expensive ones worth it?