I think it really all depends on ambient temp but....I would rather be forcing air, at a very high rate, regardless of noise (which can be dealt w/), over the heat sink. As opposed to relying on the inlet of the delta fan to draw air across the entire heat sink which I don't think will happen. Air comming into the delta inlet would probably cover the top 2/3 of the heatsink.
When the fan blows onto the sink....there is air moving across every copper fin and on the baseplate...copper is dependent on this forced air to aid in the dissipation of heat from the fins.
Heatsinks like the Alpha Pal 6035 and the Hedgehog that are designed to have the fans sucking come with a shroud that blocks the top half of the fins. This ensures that air comes in the bottom and is drawn up. Without this shroud it is not nearly as effective. If you want to experament you could use some tape to cover the upper part of the fins.