Oh Jim, it's wool for me everytime
You'll first need to ask some questions.
Although it's my least favourite fibre, polypropylene/olefin is essential if there are incontinents or untrained animals in the house. But it's awful for people like the Chinese who cook with hot oil. It sizzles and splashes and soils this fibre rapidly from both walk off at atmospheric soiling.
Because polyprop is a hard, stiff fibre, it flattens hard, hooking over at the end and sock fluff can stick to it like velcro. A nightmare for both the householder and us to keep looking clean.
I've never liked polyester and we don't have much of it over here. Crushes easily but unlike polyprop, stays soft. Cleans fine though.
All things being equal in carpet style and construction, I'd go for nylon. Type 6.6 is a little stiffer and harder to dye than the type 6, so is more difficult to stain too.
Solution dyed is even better than topically dyed.
If the customer wants a multi colour pattern, then it will probably be a printed type 6 nylon.
If the customers chosen carpet doesn't have a protector, IMO it's essential, so there's a sale for you.
Advise them to ignore any advice that a carpet can be cleaned with bleach (Clorox?) That's a hell of a problem they could be creating
Safe and happy advising
The JAGUAR 6.6 --"A TRUE INDUSTRIAL SYSTEM"; not a generic squirt & suck!