When green they both sink easily and will last underwater. Some say oaks release tannins into the water which may effect fish. I have not seen that problem during the many times I have used them. Pines make the water more acid. Oaks are harder and I think last longer. Either will work fine. I would use some with a root ball with dirt on them if you have them . Put the root ball in the deep water area. If you have a few stumps with root ball and dirt put them about 5 ft from the bottom of the cliff.