Buying mattresses is a long term commitment, I think just looking at the cost and quality is not enough. You must also know how the mattress is being constructed to be assure that it will last for long. The mattress that you buy now may looks firm and good, but who knows it starts to give you problems after a year?
In buying mattress, I think it will be good if you can get the following information from the sales staff:
1) What type of springs is the mattress using and what's the plus and minus points of this type of springs.
2) How the springs is being connected to each other to form the mattress.
3) How many springs is used in the mattress. I presume the more the merrier cos spring will be smaller so you won't feel it so much after the mattres start to deteoriate in future.
4) What's the warranty of the mattress. Remember, warranty and guarantee is different. For guarantee, the manufacturer will have to replace the defect FOC. While for warranty, the manufacture onlys agrees to repair for you with repair charges.
5) What type of fabarics is the mattress using and is there and special features of the fabrics like cooling, stop bacterias and so on.
6) Question about the manufacturer. Ususally local manufacturers are preferred. If there are any problems with your mattress, they can rectify for you quite soon. Most of the brands do have local manufacturers but in actual fact the prodcution has already shifted to other countries, so do check with the sales person where is the mattress made form. If possible, do take come chance to visit the factory if you can.
I can't tell you much about Magic koil as I am not sure what kind of quality you are looking at. Sometimes a more expensive mattress may have similar feel with a cheaper one. It is the fabrics and padding that contribute to majority of the feeling. The mattress construction will determine more on how long lasting it will be.