Our Sagging Mattress

I really need to do something about our sagging mattress. It is getting uncomfortable every night. I have to act now before I developed back pains and avoid more sleepless nights. I have been looking online and I just can decide which mattress to choose. There are so many choices and selections. I am looking for a mattress that is comfortable and have high resiliency. It is also better if it's hypoallergenic for our protection. I think something like this one below will be very nice.