Both should work ... but one may work better than the other depending on the bullet profile and plug profile .
The wadcutter plug will press on the flat spot ... the RN plug will press on the bullet sides , the round part ... but if not a perfect fit , may leave a ring around the bullet ... and that looks Wonky , but does little damage ... I don't like ring around the bullet .
Try both and see which you like ... I usually go with flat / wadcutter plug .
Don't forget you can custom fit the plug to the bullet shape with a small ball of epoxy putty ... the putty will conform to the bullet profile for an Exact Fit !
Gary