All of the mattresses you are considering use good quality materials and have no “weak links” in their design so which one is best for you would depend on all the parts of your personal value equation that are most important to you including the price of course and the options you have to customize a mattress both before and after a purchase.
I would make sure you know the type and blend of latex in each mattress you are considering so you can make more meaningful “apples to apples” comparisons.
You can also read more about natural, organic, green, and safe materials (all of which have different meanings) in post #2 here and the posts it links to so you can also make more meaningful comparisons outside of just using the claims of a manufacturer.
A forum search on Habitat (you can just click the link) will bring up more information and feedback about them but for most people they wouldn’t be in the same “value” range as either MyGreenMattress or FSF or many of the other options that are available to you as well.
OMI also uses good quality materials but they are in a much higher price range compared to many other mattresses that use the same type and quality of materials so for most people they wouldn’t be in the same “value range” either although some of their mattresses do have an organic certification for the mattress as a whole if that’s important to you.
I’m not familiar with the detailed specifics of each cover so you would need to talk with each of them to find out any details of the cover and the quilting materials that are used in each mattress you are considering (such as the type of fabric and the type and amount of any quilting) but making meaningful comparisons between covers is not something that you can do just by knowing the specs of the fabric or the weight of the quilting materials because there are many more variables than just these that will affect how the cover contributes to the feel and performance or a mattress. You may also want to ask if any of these mattresses use only wool as the fire barrier or if they also use another fire barrier system as well (which may mean less wool in the quilting).
I’m not sure what you are referring to with FSF’s “mattress pad” but in very general terms adding a mattress pad to any mattress will affect the feel and performance of a mattress yes. There is more about mattress pads that can be added on top of any mattress in post #10 here. There is also more about wool quilted covers (just in case that’s what you mean) in post #6 here.
There is also more about organic certifications in post #2 here and the posts it links to that can help you compare the certifications for the materials in each mattress and the mattress itself. The only one of the three you mentioned that make mattresses that are certified as organic as a “complete mattress” (vs only the materials in the mattress) is OMI but this would only apply to their mattresses that only use organic Dunlop inside and have organic covers. Many of the OMI mattresses use Talalay and there is no organic Talalay in the world.
I’m not sure which specific mattresses you are comparing but it would depend on the design of each mattress, the type of materials inside it, the details of its construction, the wholesale costs of the materials, and on the margins and pricing policies of each manufacturer (among other variables as well). Once you have confirmed that the materials and design of two mattresses you are comparing are the same and you know you are making “apples to apples” comparisons between mattresses that have a similar design and type of materials and components then differences in prices would depend on the many other variables that are discussed in post #14 here. Of course the “commodity value” of a mattress based on the cost per inch of latex or the price per pound of fabric or quilting materials will have little to do with which mattress is the best value for you (see post #13 here).
I would keep in mind that there are many types and blends of latex that each have different costs (see post #6 here) so part of the differences may be differences in the type and blend or the amount of latex in each mattress.