Hi misha,
You have a whole fleet of questions there 
To start off with, if you haven’t, it may help to review the post on how to select the best mattress for yourself. It may help you measure both mattresses easier and help in decision making. Phoenix does a wonderful job with it. You can see it here:
https://forum.mattressunderground.com/t/how-to-look-for-and-find-the-best-mattress-for-you-read-first
The magniflex appears to be a memory foam mattress, and the Sleeptek is a latex mattress. Generally memory foam and latex are described as being at 2 opposite ends of a spectrum. They behave and feel quite differently. It’s a preference which you prefer.
The memory foam in the magniflex appears to be a standard chemical based memory foam (all memory foam is). The safety of memory foam (off gassing, chemicals), is often established by CertiPUR, but they seem to indicate on their website they adhere to a slightly more stringent requirement called Oeko-Tex 100. Unfortunately, I can’t find a document on their website on what materials they have Oeko-Tex 100 certified. But assuming it’s all materials/ the entire mattress, then that is usually a strong indicator of safety.
If you are looking for a “natural” product, then the memory foam is by no means a natural material. But for everyday standards (assuming the entire magniflex mattress / all materials within are Oeko-Tex 100), both would be considered safe. If indeed the memory foam is Oeko-Tex 100 certified that’s a higher standard than usual. You can read more about safety and certifications here:
https://forum.mattressunderground.com/t/mattress-question
As to the durability of each, the sleeptek uses a very durable Dunlop latex. For the memory foam, or any other polyfoam, you’d want to know the density. I didn’t see the density documented, but it didn’t look super hard. The first post I linked to has info about the durability of materials. With memory foam, good material starts at about 5lbs.
From personal experience (my wife has horrible allergies to dust mites), we have a green sleep mattress (similar to the sleeptek), with all natural wool and latex. We still use an organic cotton dust mite case for the mattress, and pillows (also all natural latex and wool). She finds the covers still help.