Hi zagestone,
The choices you are listing are very much apples to oranges comparisons.
You can see a bit more about comparing memory foam to latex in post #2 here but I would urge you to compare the two materials in person so that you are more familiar with how they generally compare. Any comparison of this type would be partly objective but partly subjective as well and relative to each person’s perceptions and preferences.
The budget saver has a bonnell innerspring instead of polyfoam which would have a different feel compared to a polyfoam support core and there are also some other diffferences as well (such as the Ultra Plush would have a quilted cover while you would be more directly on the latex in the budget saver / topper combo with some softer polyfoam underneath the latex instead of over it). You would also be dealing with two variables instead of one because the comfort layer you chose for the mattress would interact with the topper and trying to figure out how the specific combination would feel to you (one of the two comfort choices in the budget saver along with the ILD of the topper you chose) would be much more unpredictable than only dealing with the single variable of the ILD of the latex itself. There would also be no real price advantage and the budget saver/topper in queen size would be $269 (mattress) + $329 (topper) = $598 while the Ultimate Dreams by itself would be $599 so in price terms they would be equivalent. If you added a gel memory foam topper over the budget saver then the cost comparison between the budget saver/topper would be $269 + $189 = $458 and the 10" gel memory foam mattress with the same layer of gel memory foam on top would be $499 so there would be some advantage but again there would be more unpredictability in terms of how the topper interacted with your firmness choice and you would have an innerspring/gel memory foam hybrid “feel” instead of a more typical memory foam / polyfoam feel.
I don’t know the density of the foam in the budget saver (at this budget level most people wouldn’t be concerned about foam density although it would still be good to find out) so I don’t know how they would compare in durability terms. The only other difference is that with a separate topper you would have the chance to replace it if you wanted a different firmness choice either initially or down the road without replacing the whole mattress.
All of these are very different options that can’t really be directly compared to each other in terms of design and what I would suggest is that you talk with them directly about how they may compare in terms of feel and performance because if anyone would know they would.
Phoenix