When you "rent" points, you are simply making a reservation in the name of another guest who is NOT a
DVC owner. It's wise to have a written agreement of some sort which includes payment terms, cancellation policies, etc. Since the person is not a member, you are their lifeline to DVC. You need to call and check on availability, make the reservation, forward a copy of the confirmation, and so on.
When you transfer points, you are dealing with a fellow DVC member. In a transfer situation, the two parties simply agree on payment terms, money changes hands, and then the points are moved from one member to another with a single phone call.
One limitation I can think of for transfers is that you can only transfer points in one direction each year--you can either RECEIVE points from another member or GIVE points to someone. You cannot do both in a single year.
Points are supposed to retain their original Use Year and Home resort even when transferred.
All things considered, transferring points is a cleaner transaction.