What type of Cat 6 cable to run behind basement studs?
Riser should be fine unless you expect it to really get wet, in which case a waterproof version might be wise for a bit more money. Since you are using inside, a waterproof cable needs to be an &quot;indoor/outdoor&quot; rated cable - strictly outdoor cables are not tested for and may not meet indoor smoke ratings, and are limited to 50 feet from the point they enter the building.
We run plenum or riser attached to concrete all the time in industrial work.
I still recomend conduit for the reasons I always recommend conduit for networking (future-proofing and rodent-proofing (EMT), or at least resisting for the plastic stuff.) Perhaps metallic flex conduit if EMT is too daunting and you'd like tooth-proof. Comparatively expensive, though.
However, since it appears that you could run new cable as easily in the future as you can now, you can probably get away with just doing that. I would suggest using some cable ties with screw holes to support the cables by attachment to the studs, where accessible, rather than just draping them on the floor the whole way.