NSP (which stands for Non-Sulphur Plastilene) is a line of professional, sulphur-free sculpting clays. It can be melted and poured at approximately 85˚C. However, it is not as reusable as Monster Clay. And it sticks to the tools a lot. But, it is cheaper, about £7 for a block.
The colours do not exactly determine its softness/hardness. From what I know, the medium grade clay in UK only comes in Green. I contacted a few retailers and they all gave the same answer: Green.
NSP clay is widely used by sculptors. Very versatile clay. Just need a little heat to work it, or even if you knead it, you can soften it with heat from the hands.
As for what grade to use for sculpting, depends mainly on the temperature at your workspace. A NSP hard clay which is almost impossible to work with in a cold country, might be very pliable in a tropical country. There was even a sculptor that commented that, due to the vast differences in temperatures between seasons, he uses different clay in summer and winter.
When heated up, unlike the Monster Clay, Chavant NSP gives out a unpleasant chemical smell, so do not overheat it.
Beautiful textures can be created by carving into it, when it is hard enough. However, when it comes to smoothing it out, a good thing to do is to dust talc powder on, so that the little clay bits can be dusted off with a brush. I do not have that sticky problem with Monster Clay.
After reading lots of reviews, and my own hands-on experience, I do not think that Monster Clay is totally superior over Chavant NSP. But it will be much easier and straightforward for beginners to use Monster Clay, as it is much more hassle-free, and 'cleaner'.
Image of sculpt of Dick Smith is from here.
Image of 'A little mischief' by Art Farmer is from here.
Image of Batman Suit is from here.