Hi laurbuzz,
Both of the mattresses have the same thickness of latex in the comfort layer and both are sewn shut so from this perspective they are similar. The type of material in the support layer can make a bigger difference than the thickness of the layer but in this case the type of material is the same (as fr as I know) so It’s true that the extra 2.5" in thickness of the support layer may make some difference in the case of heavier weights (assuming they are both the same type and density of polyfoam). There is more about this in post #14 here.
The key is “thick enough” rather than “more is always better” so the Ultimate Dreams along with a topper would give you a 13" mattress which would likely be thick enough … especially with most of the top 6" being latex. One other advantage would be that dealing with Chuck at Amazon would probably provide you with much more useful advice than dealing with Overstock who are not exactly “mattress people” and won’t be able to offer you the same type of advice about their mattresses.
In any case … I don’t think that you could go too far wrong with either choice and a topper.
The only other thing I would consider is that with the Nuform and a 3" latex topper you are also getting close to the price of an all latex mattress which would certainly be a higher quality choice yet and could offer options that aren’t available with either of the mattresses you are currently considering.
Phoenix