I'm in the "BOTH" camp as well.
The majority of my cuts are done with a straight blade. For some text, especially if it is near the bottom or top of a cutting, and cursive, I will use a spiral.
I also use spirals for widening features (Veining).
I don't like them on solid timber, as Jarrah is to hard and it takes forever and Pine leaves heaps of "Fuzzies"