The level of comfort and support you need is a very personal decision and you should allow plenty of time to test and compare different beds. Do not be embarrassed about lying on the beds in the showroom, that is what they are there for! If you share a bed with your partner you should always test the bed and make the decision together to ensure maximum comfort for you both. If you have different support needs, it is possible to have two single mattresses of different tensions zipped together. Some manufacturers can combine different degress of firmness in one tension.
To test a bed, make sure you are wearing comfortable clothes, take off your shoes and outdoor clothing and lie down on the bed on your back. Slip your hand between the base of your spine and the mattress. If you can easily do this then the mattress may be too soft whilst if there is a hollow the mattress may be too firm. Ideally your hand should be a snug fit. It is important to adopt your normal sleeping position and lie there for several minutes to test your support. Also, when you lay on your side your spine should remain horizontal and you should be able to turn easily.
Remember that taking the time to try a new bed is a 10 minute investment to ensure you get 10 years of comfortable sleep.