With regards to comparing GreenSleep to Natura its pretty different. Natura does not offer any innerspring mattresses that are near the quality of the Green Sleep models...in truth the only other brand that I know of that does an all latex pillow top in conjunction with a spring system that can really compare to what GreenSleep has availabe is Carpe Diem, although they are not a certified organic mattress either (although still natural).
I would not say that the GreenSleep foundations are absolutely required but it is one of the things that really sets them apart from the rest of the crowd. All of the worlds best beds have flexible base systems, whether or not you will notice a tremendous difference is hard to say. One thing is for sure when you look at the mattress only pricing they really blow away a lot of their competition in terms of what you get for the price. The way those coil beds are constructed is pretty labour intensive, the quality of the wool, and cotton in particular are much higher quality than what you typically find in any mattress, and their rubber is supposed to be some of the best if not the best around. Although that is somewhat subjective as some will find softer talalay latexes to simply be more comfortable (depends on preference).
Natura is a pretty big brand these days but they are less focused than GreenSleep. GreenSleep does not do any advertising and most likely never will, they exclusively build high end natural beds, where as Natura builds good quality mattresses with a hugely varying amount of Natural content. Both companies are quite reputable. I think if you are looking @ the higher end and comparing apples to apples in terms of pricing than the GreenSleep quality is easily enough to sway me into their favour.
I should mention though that I do sell beds and carry both of these brands. I don't carry Carpe Diem but they are as I mentioned the mostly directly comparable bed out there, so if there is a dealer near you they may be worth looking at.