Like insurance sales in North America, mattress selling is based on guilt - in this case guilt that you haven't bought your loved one, who is going to spend a third of her life on it, the most expensive product in the shop.
Get a platform, and a mattress that feels reasonably comfortable in the shop, and you are done.
If you don't like it, or if it sags, you can replace it three times for the ludicrous price of some of the hyped up gear out there!