The "Poly" will be better for most situations Russ, being the stiffer of the two. The other one will be very soft so it won't get into any polyprop carpets, cut pile or low profile, it'll just skim the top of them which isn't really any good.
But why not get both? They will last forever and a day, once you have them you have them!
The softer one will be excellent on wool & nylon, in the right situation.
When you get your brush(es) don't forget to run them in on a bit of concrete or paving slabs, stick some water down and just run the machine on it with the brush attached, I'd recommend at least 30 minutes. Doing a few hard floor jobs with the brush first will have the same effect