Personally I wouldn't get a 100% polypropylene. It looks fine to start with and it certainly wont wear through, however, the pile flattens very quickly and it loses it's bounce. I think you would regret it. An 80/20 won't shed as much as 100% wool - I think you would be better off with that. I am in the process of changing my carpets at the moment. we have some 100% wool ones on the stairs and they have worn through and do shed fluff (9 years old) but I don't think the 80/20 are as bad. We replaced the play room carpet a couple of years ago with a polypropylene one and it looks crap now, its all flat and it's fraying at the door bar (that is probably due to poor fitting though) to be honest we got it through one of these big carpet stores and it was a total rip off. Now we are having to replace it again. We are going with an independent fitter this time and ordering our own carpet. We are doing the entire upstairs and the stairs and the playroom. It is costing a lot less that the big shop wanted for the cheaper carpet and all the extras such as gripper and door bars etc are getting thrown in for free.
It is working out £28 a sq meter for 80/20 wool twist 50oz carpet, top of the range underlay, gripper, door bars and fitting.