Hi Sherryleb79,
I’m not so sure I would agree that it’s a “really nice mattress” since it has 3" of 3 lb memory foam which is a lower quality material than I would consider in your budget range.
I would look for better quality than the Serafina not the same quality.
Firmness and softness are very subjective and what is too firm for one person may be too soft for the next (or vice versa). The only two ways to choose the mattress that is best for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) is either with your own careful and objective testing at a local store or a more detailed conversation with a knowledgeable and experienced online provider that has your best interests at heart and can help you choose which of the mattresses they carry has the best chance of being a good match for you (see mattress firmness/comfort levels in post #2 here).
I would encourage you to follow the steps and guidelines in the tutorial post.
Phoenix