So far, whenever I decon a car, I ALWAYS Clay and Deironize spray the vehicle...would there be any reason you ever do one or the other instead of both?
I have done a couple of new car preps that have only required a deironize services, but these cars came straight from the dealership to me (family vehicles).
If anything, I would most likely do both or just clay, depending on the paint condition and color. On darker colored cars you could skip a decon if it's not that bad, the color will mask the iron deposits, but on lighter color cars it makes a much bigger difference. Claying or using an clay-replacement product is quick and easy, I see no reason to not do it. Decon can get expensive doing larger cars/trucks.
If I am applying a coating I will always remove iron deposits as I dont want to lock them into the paint. I will clay or Nano Scrub Pad a surface if it is still not smooth to the touch. Since I only do my own vehicles I pretty much do both at least 1x per year when I re-up the protection. This is the first year I am letting the Opti-coat run as it is for 14+ months. Thinking about just hitting it with Iron X next wash to remove any contaminants. Good Luck.
Most cars are both. Not every car needs clay but some new car preps or extremely clean vehicles you can get away with just iron decon as long as you wipe each panel after soak and then wash. If I am opting to not clay I will iron decon twice and if I see no transition to red on particles the second round I know the finish is good to go.