Hi shah,
The mattresses you linked are very difficult to compare because they are all very different from the Insight and are really an “apples to oranges to peaches” comparison. They all have very different layering which will feel and perform differently. Besides the differences in the layering of each mattress, memory foam mattresses are especially difficult to make comparisons because there are hundreds of different types of memory foam and each of them can feel and perform very differently from each other. This is made even more difficult because of the lack of “comfort specs” in the mattresses (how soft and firm each layer is).
There is more about trying to “match” mattresses in the beginning of post #2 here and more detailed information yet in post #2 here.
In terms of quality and “value” … there is no doubt that the Ultimate Dreams is a better choice than the other two but you can’t feel quality and value and it’s just as important that a mattress matches your needs and preferences or what I call PPP (Pressure relief, Posture and alignment, and Personal preferences).
Dreamfoam has said that their gel mattress is similar to the Tempurpedic Cloud (but a little firmer) so this would give you some sense of how it may feel for you since these are widely available but even here … different people can have different perceptions of the same mattress depending on their body type, sleeping positions, and perceptions.
If I had to guess … I would say that the Ultimate Dreams would be softer than the iComfort insight. At least generically … the layering in the Ultimate Dreams would be closer to the iComfort Savant but even here the layering is different and the foams would not be the same.
I personally wouldn’t even consider the Serta 10" mattress you are looking at because the odds are very high that the quality of the foams are even lower than the iComfort and there is only 1.5" of memory foam in the mattress and the rest of the comfort layers (and the support layers) are cheap low quality polyfoam.
There is more information about foundations and boxsprings in this thread including some links to some better quality and low priced options.
The better options in the Greater LA area that I’m aware of are in post #2 here. You have some very good choices in the area and some preliminary calls and conversations describing what you are looking for (a memory foam mattress that is similar in construction to the 3" of memory foam, or gel memory foam, over polyfoam of the insight) should help you narrow down your options.
Phoenix