Buy the best mattress you can afford and change it every 8-10 years. The sales people are right when they say cheap mattresses won't last.
There are different types of springs in sprung mattresses and the best is the pocket sprung mattress where each spring sits in its own pocket which means it's more durable and you get less 'roll together' (where you and your partner end up in the middle of the bed whether you want to or not) and you get less disturbance when your bed partner moves during the night.
There are latex foam and memory foam mattresses which also achieve the same thing and are probably better for allergy sufferers.
Or if you prefer a soft mattress and your bed partner likes a harder surface, go for a Sleep Number bed where you can alter the firmness to your needs.
Water beds have moved on since their days in the early 80s when they were just bags of water.
I run a sleep site and there is plenty of information about sleep, beds and bedrooms on there - sleepbetterlivebetter.net if you wanted to look.
Good luck and don't forget to set aside plenty of time for buying a bed or mattress (or both) - it's an important piece of kit!