A sprung edge base does act like a shock absorber, but it depends how sensitive you are as to how much you will notice a difference in feel.
A platform top will definitely give any mattress a firmer feel compared to a sprung edge base.
You should go with the bed that gives you the most support and best comfort level, irrespective of price tags. Remember that taking the time to try a new bed is a 10-minute investment to ensure you get many years of comfortable sleep. Most retailers will not exchange a bed if it proves not to be comfortable, so it’s very important to get it right first time.