Hi jethrodesign,
Both Mattresses.net and Flexus comfort are members of this site which means that think highly of both of them and they are both knowledgeable and reliable manufacturers that provide great service to their customers.
There is more about the differences between different types and blends of latex in post #6 here but 100% natural Talalay is a little more costly material and will be a bit firmer and more “supportive” and heavier but also a little less pressure relieving than blended Talalay in the same ILD but many people wouldn’t notice much difference with only a 3" layer if the ILD was the same.
The 22-24 ILD would be a bit firmer than the 19 ILD both because it’s a higher ILD and because it’s 100% natural but they are both in the “soft” range at least by most people’s definitions of soft.
They both have good quality covers but the covers with Flexus are zip covers and the cover with the Rejuvenite doesn’t have a zipper.
If you like the feel of Talalay latex and a 3" topper would be appropriate for you based on your mattress and the topper guidelines (“a fair bit to a lot” of additional softness and pressure relief) then both of them would work well “on average” for most people in average weight ranges so your choice would really be based on “best judgement” and on how you feel about the difference in the blends of the latex, in the firmness levels, and of course on your conversations with each of them on the phone.
Phoenix