I've built a LOT of tubing parts. Notched thousands of notches.
I have been very happy with the JD² Notch master for the occasional notch. We recently built a roll cage in an RX-7 with it as well as a stainless steel pole arch for a charter fishing boat. I found the Milwaukee Hole Dozer to be the most long lived hole saw. You want to use a good triple reduction corded drill. We burned up a few Milwaukee Fuel cordless drills at the race shop on the notcher. Use a corded drill.
If you are going to do a LOT of notching, an abrasive notcher is very fast and more tune-able. Than a hydraulic bladed deal. When I worked in an off road shop that did mostly tubing, we had a Peddinghaus belt notcher.
Ironworkers and blade style notches are fairly crude, and the tooling is expensive. Its hard to not distort really thin wall tube.
You can also notch tubing with an annular cutter or hole saw in a milling machine. If you have a machine with enough nuts, I have had good luck with large roughing end mills. I used to use a Cincinnati horizontal mill with the tube clamped to the table and the cutter in the horizontal spindle.
Mittler Brothers makes a notcher that uses mill cutters.
The best answer, of course, is a tube laser. But, that requires you to have a CAD drawing, as well as a vendor with a tube laser. We used to get parts cut by the hundreds, all identical notches cut as well as etched bend lines for production bending. It is the most expensive option for small runs, but once you get past the set up fee, the parts get really cheap once you factor in labor.
