Hi Sepia,
Both innersprings and a firmer latex core can be used as a support layer and each has very “different” characteristics but besides the more obvious ones the most important differences are the ones you can feel and that you personally prefer. Both of them come in softer or firmer versions and in many different designs so an innerspring could be firmer than a latex core or the other way around depending on the specifics of the components you are comparing. There is more about the 4 main types of innersprings in this article and in post #10 here and more detailed information about innersprings vs latex support cores in post #2 here and more about the different types and blends of latex in this article and in post #6 here.
Some of the more technical differences include …
Innersprings have a more “flat line” response curve than latex (see the graph here) but because there are many types of innersprings with different response curves this doesn’t always hold true and there are also different types of latex which would also have different response curves.
Innersprings absorb less energy than latex which means they are more resilient. They “push back” more strongly than latex in other words but this is not the same as softness … only about how much of the energy that is used to compress them is lost (or how high a ball will bounce when it’s dropped on them).
Latex has a similar or higher compression modulus than most innerspring spring rates and either gets firmer with deeper compression at a similar rate as an innerspring (Talalay) or at a faster rate than an innerspring (Dunlop) which means it can be more “supportive”.
Different innersprings have widely different abilities to take on the shape of the body (depending on the number of coils and how independently they function) while latex is much more “point elastic” than any of them because it can flex in each part of the core with less effect on the area around it than an innerspring.
In general latex will be more motion isolating than an innerspring.
Firmer latex will be more durable than an innerspring but neither of them would tend to be the weak link of a mattress.
Innersprings have more “air” in them so they would be more breathable than latex even though latex is the most breathable of the foam materials but the deeper layers of a mattress also have less effect on the ventilation and temperature of a mattress than the comfort layers.
Most latex is more expensive than most innersprings.
They “feel” very different with innersprings being more “bouncy” or “springy” than latex (although latex has more “spring” than other foam types).
There are many other more technical differences but the most important differences are the ones you can feel. Either of them can make a good choice for a support layer and in the end it really boils down to which one you tend to prefer. Both of them can provide good/support alignment. There are so many varieties of both that it’s not really possible to make more specific comparisons outside of some of the more obvious and more “generic” differences that I’ve mentioned.
Making more generalized assessments of a mattress only based on one component doesn’t take into account that all the layers in a mattress work together and will affect its feel and performance so either one could be part of a mattress that provides you with your pressure relief and alignment/support needs. Assuming that all the materials in a mattress are high quality … everything boils down to which mattress design works best for the two basic functions of a mattress.
I think that all of these could make very good choices. Once you have narrowed your list down to choices between “good and good” … my job is done and it’s time for me to “step out of the way” so you can decide on which is best based on your conversations with each manufacturer, your own personal risk tolerance (online is “riskier” than local), and the other parts of your “value equation” (see post #46 here) that are most important to you. I would never make a “final decision” for anyone else because that’s when it needs to be up to your own unique preferences and what is more or less important to you.
The good news is that there are no “bad” choices in your list and your own thoughts and confidence about which will best “match” what you need and prefer is the only real way to choose between them. I know from personal experience how difficult it can be to “eliminate” a great choice but it has to be done (unless you buy more than one mattress ;))
Thanks for the thoughts … and it’s nice to see that the site is reaching a place where I can “begin to breathe” and there are a lot of things that I have planned in the next year or two that will increase the scope and helpfulness of the site. 
Phoenix