depending on certain factors...

1. Your existing bed frame can put in how thick the mattress. Some divan headset is fixed and changing mattress thickness means change bed frame too. For mine, adjustable, can put low like 8" thick to 16" thickness. Thicker mattress means your bedsheet gotta be changed to cover the new thicker mattress.

2. How hard you want your mattress to support your back and body weight. Older you are the more firm it should be. 1200 pocket springs feels better than 800 pocket springs, the more the better.

3. Mattress material. Coconut fibre, latex, hybrid + spring, etc etc. You have to try it out yourself and which is comfortable for your sleeping position. Not everybody feels the same. I settled with bamboo yarn material with 1200 pocket springs instead of full latex.

4. Price.

In the end you have to try it out yourself to know what feels right for your body. Be smart to separate the information from sales talk, different shop has different sales margin and they will push you with those they can get the most commission.

good luck, let your body tell you which is good.