Conclusion

If you want to buy a new mattress keep these things in mind:

Do the shop have a trial period for the mattress so you can try it at home?
If so do they give a 100% money back garanty if you dont like the mattress?
Go for quality! You do not need to buy the most expensive mattress, but do not buy the cheapest one either, your rest and your health is something too important to compromise on quality. An average mattress should be at least 16cm thick but I prefer thicker ones.
In principal a thicker mattress offers more comfort than a thinner one.

Another aspect is the warranty, how is the bed put together, is it fully guaranteed or is it a limited guarantee, what exactly is the guarantee on, for example is it on the foam or is it also on how it is glued together? You must be wary of people trying to pass off poor guarantees; sadly in many cases they make you have to prove something is wrong with the bed rather than that it is just not the right choice for you.