My decision for tempo type used on a game by game basis is based largely on lessons learned from vet's posts in the forums. However, there is one additional factor that I use, which I don't see come up a great deal in the forums; and that is the idea of IQs, which is often the tipping point for my decision on tempo.
The increased possessions only have value if they can be meaningful (no duh, right?) I guess my point is: I concern myself with thinking about TOs/fouls generated via tempo, which I believe to be primarily influenced by IQ.
I find that I rest on using normal most of the time anyways, as gill suggests to, it's only in small cases will I use slowdown and even more exetreme cases do I use uptempo.