I have bought the £100 one as it's the same one I had for my DS 4 years ago.
I went for that one as if baby is ill or leaks onto one side of the mattress you can use the other side of the mattress while the removable cover is in the wash.
Also, the mattress lasted until my DS was in a full sized bed (at 3 1/2) without showing much wear and the springs still seemed really bouncy too.