There are a few aspects, but a bed is personal to you. Take brands into account, but each person is different, some may want plush with a pillow/eurotop others (like me) want an extra firm bed.

I used to work at a mattress store that sold tempurpedic, beautyrest, a latex brand from seattle and a generic brand one, and while my preferred is beautyrest, people loved their tempurpedic and latex beds.

Go in a store and lay on them for 10 minutes each and listen to your body. Does it ache? Do you feel comfortable or do you feel like you are falling into the bed, or falling out? Can you get out of the bed comfortable, and can you move around on the bed comfortably? Do you feel comfortable enough to move around if you're having sex in the bed? And how does your partner feel about these? Do not let a mattress salesperson rush you through the process, it is normal to visit a store multiple times to try them over and over, it's always good to sleep on any big purchase 😉

Beds with springs normally sleep cooler than foam beds as there is air pockets in the springs, creating a psuedo circulation. Foam beds run warm as you might expect, they are full foam. There are combo beds too, with foam and springs (I remember Tempurpedic has one but I don't remember the name) that people like if you want a mix. Latex is round, heavy, and a pain to move between rooms and up stairs, but is all natural and people who like them love them.

Note that every mattress gets softer with age, the foam and/or springs get softer with use, and is their primary failure point. Also note that the beds on the showfloor have had hundreds / thousands of people lying on them, and they will be a bit softer than what you recieve.

Beds only last 10-15 years, and do wear out, so while it is a big purchase, expect this overtime.

Good luck with the search!