I bought the larger HF needle scaler years ago and used it for chipping welding slag. One time I used it to chip 3/8" of rusty scale from an old cast iron rendering kettle that held about 50 gallons of water. That was a BIG job, and it didn't miss a beat! I still have that tool and use it occasionally, mostly on heavy rust scale. It still works fine.
I bought the smaller HF needle scaler because the larger one used more air and was heavier/bulkier. The smaller unit works well on weld slag and surface rust. It fits into smaller areas and I use it a lot. I run 6011 and 7018 rod and this tool cleans up the weld nicely. It does not seem to use much air.
My use is repairs to equipment around a large tree farm, so it would be "Light Professional" use, not daily production.