Reply by DS

Tear out and application usually will determine the best type of spiral to use. If you are using a bit in a panel router (or router table) to cut dadoes in melamine, a downcut spiral will shear the melamine cleanly with no tearout at the edges. An upcut will run cooler, since it clears the debris easier, but it will tearout the laminate.
If you are routing a fairly hard, dense solid lumber, the cooler upcut may be preferrable since tearout would be negligable—though I would consider a chip breaking up-spiral cutter so the debris will clear easier.

In solid carbide there is also something called a compression spiral. This has an upcut for a portion and a down cut for the remainder of the cutting surface. This is usually used to cut two sided melamine parts on a cnc router, or with a collar and templates and jigs.
Since the debris is slightly compressed at the middle of the bit before clearing out, this type runs hottest of them all, but can cut cleanly at the top and bottom with no tearout.

-- "Hard work is not defined by the difficulty of the task as much as a person's desire to perform it.", DS251