Hi amindu,
The first place I would start your research is the tutorial post here which has all the basic information, steps, and guidelines that can help you make the best possible choices.
Outside of your own personal testing and experience for PPP (Posture and alignment, Pressure relief, and Personal preferences) a mattress is only as good as its construction and the quality of the materials inside it.
Unfortunately … neither of the Costco mattress descriptions you linked to include any meaningful information about the quality of the materials inside them so it’s not really possible to identify any weak links in either of them or make any meaningful comparisons with other mattresses unless you are able to find out the specifics of all the layers and components inside them (which is unlikely with major brand mattresses). In general I would avoid major brand mattresses or any mattress where you aren’t able to find out the quality of the materials inside it or where the odds are high that the quality/density of the material is relatively low (such as the polyfoam that is used in both of the mattresses you linked).
I would also tend to avoid purchasing a mattress with the intent of adding a topper unless you have no other options or have tested the combination in person because that introduces more uncertainty and variables than just buying a mattress alone that is already a good match for you in terms of PPP. If you do decide to go in this direction then I would first buy the mattress and use your personal sleeping experience along with the topper guidelines in post #2 here and post #8 here to decide on a topper that has the best odds of success. I would tend to avoid both of the mattresses you are considering.
The Ultimate Dreams Talalay latex topper is a good quality/value topper and Dreamfoam is a member of this site which means I think highly of their quality, value, and service, but it would be important that the firmness you choose is a good match for you in combination with the mattress you purchase. A topper can feel and perform very differently over different mattresses.
Phoenix