If you want to get a few years wear out of a stairs carpet I would suggest going for at least 80/20 wool/nylon. Inexpensive nylon carpets look okay in showroom but they seem to hold dirt and are very difficult to hoover in my opinion. The wear and tear on a busy stairs means a cheap carpet will not last long and look shabby in no time. If you are getting a carpet runner fitted with metal rods the idea is that a quality fitter can move the carpet later to hide wear and tear. Also then finally colour is very important, I would never go for light coloured carpet. I fitted really good light coloured wool carpet in a bedroom, and it was impossible to keep clean but has withstood wear and tear. My carpet on stairs is a Navan twist carpet 80/20 in a taupe/neutral shade and after 6 years, 3 kids, heavy wear and tear is still in presentable state & clean today, as it vacumes easily with a Dyson.