I got a sprung mattress because it is supposed to last 3-5 years as opposed to foam which is cheaper but may need replacing as often as once a year. I also chose one with a waterproof layer which can be unzipped and cleaned so once baby is potty training it will be easier to clean up at night times. I got mine on sale in mothercare, there was an offer on to buy a cot and get half price mattress. If you have a mothercare i recommend going in and asking someone to run through the different types of mattress, they were ever so helpful in helping me choose which one to go for.