Don't make them too thick and fluffy because while it might make falling a bit easier on kids, it tends to lead to easier slips for adults.

We put berber carpet on our stairs- as a runner- 8 years ago and it still looks good. it is a Karastan beige. cleans well- both spot and steam cleaned- which it needs again.

Thanks for your responses! This is very helpful advice, because preventing falls is my first priority. Cooperbailey, it's helpful to know that berber has worked well for you. I am considering berber, but I haven't seen it on stairs.

If you have cats, avoid berber. They like to get their nails caught in the loops and rip it up!

We had plush wall to wall downstairs and then up the stairs to the second floor eons ago. The plush was a bit slippery in socks- so we either barefooted it or wore slippers- ( or jammies with feet)
The berber is not slippery and it wears well.