@kitten I'd say, from personal experience, get a memory foam mattress. Your not as likely to get stabbed in your sleep when the mattress gets worn down, and it just feels nice to not have metallic bits being what keeps your mattress propped up. (I got stabbed a bit when I was sleeping on a spring mattress and it gave out mid-sleep)
@kitten Well, when a mattress you've bought pre-owned at a major discount is older than you, such occurrences will show themselves as I've come to know. Shouldn't be an issue for you seeing as your buying a new one, sorry for the distressing comment.
@kitten don't forget durability! an IKEA mattress might be ok for a while but after half a year you might already start waking up in pain. we bought expensive mattresses about 6 years ago and have not regretted that!