Hi gatorgirl,
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 … and know how to avoid the worst ones.
Latex and memory foam are about as different as two materials can be (closer to opposites than similar) and your own local testing on both will give you a good idea of which type of foam you tend to prefer. There is more about the differences between them in post #2 here.
To make any meaningful comments about a mattress I would need to know the specifics of all the layers in the mattress (see post #4 here). Outside of PPP … a mattress is only as good as the quality of the materials inside it and without this you can’t identify any potential weak links in the mattress or make meaningful comparisons to other mattresses.
You can also read the guidelines I would use for a memory foam mattress in post #10 here.
I would avoid any mattress that used 4 1/2" of 3 lb memory foam (or more than about an inch or so) unless you were in the very lowest budget ranges where durability isn’t a significant issue.
I would also read post #2 here and post #2 here (and the posts they link to) before considering either the Dynasty or the Lucid.
Brooklyn Bedding is one of the members of this site which means I think very highly of them and they compete well with the best in the industry. The Eurotop latex uses a 3" layer of Talalay latex in the firmness level of your choice and you can also exchange the latex layer for a softer or firmer versions if you decide you need to change the comfort level of the mattress after you’ve slept on it. Aloe Alexis also uses very high quality materials (2 layers of blended Talalay latex and a higher quality 2.17 lb base foam and a quilted bamboo cover that includes wool in the quilting). It also can be customized for each person (with either of the two latex layers) and it also has the option to exchange either of the latex layers if you wish to fine tune or change your comfort level after you’ve had the chance to sleep on it.
Phoenix