We have had 80/20 wool mix, 100% wool, and polyproylene carpets fitted over the past four years.
Pros of the 80/20 and the 100% wool have been that there has been minimal shading and pile flattening. As you hoover the carpet sheds a little each time to keep the colour looking fresh. (we have a grey 100% wool carpet, and 80/20 carpet in a duck egg colour).
Cons of the wool mix and wool carpets have been that we have found it very difficult/ impossible to get some stains out of them, which is irritating as they are not very old (4 years), but look horrid in areas. We have cats though so quite a lot of the stains have been due to the cats being sick on the carpet while I'm not around, or bringing a creature in and dismembering it on said carpet... The cats also choose to scratch on the wool carpets rather than the polypropylene carpet, (or several scratching posts...) This ruins the twist in the twist pile 80/20 carpet, and just makes it look like a fuzzy patch.
Pros of the polypropylene carpet have been ease of cleaning stains which have come out far better than the same types of stains on the wool carpets. The cats aren't interested in scratching it either, and we picked a polypropylene carpet that looks a little bit like an 80/20 wool carpet which I prefer the look of. It was cheaper than a good quality wool mix carpet.
Cons of polypropylene have been that I can see shading on it, especially in the doorway, and the pile has definitely flattened in a more pronounced way, and more quickly than the 80/20 wool mix carpet that has been down for twice as long. We have a mid brown/grey PP carpet with flecks of lighter grey throughout, and though it is darker than any other carpet we have in the house, it does also look grubby in places where there is more footfall. I haven't hired a carpet cleaner or hired someone to clean it though so I don't know if it would come up well if it were cleaned more thoroughly than hoovering.
From the above, we will be sticking with polypropylene for future renovations I think, just for the ease of spot cleaning stains and the cats not scratching it to death. If we didn't have cats, or young children, I'd choose 80/20 wool mix carpets, purely because the pile does not seem to flatten so much, and there was more choice of wool carpet colours than polypropylene when I was looking.
If it was not stained or scratched, I think our 80/20 carpet would still look nearly new though. Our polypropylene carpet certainly does not look nearly new after only 2 years!