Its all about the budget. Chemically dipping and then having them prep/prime the frame with a sealer is the way to go.
Sand blasting works fine. The frame is heavy steel, so not too much of a worry in warping panels. The down side is that you cannot get inside the boxed areas. I sand blasted my frame myself. Light rusting throughout, and cleaned up real good with little effort. I did the frame in "thirds." Once I got one section blasted, I wiped down and then used the POR-15 on it. Then did the next section.
For inside the frame, you can get some sand blasting in there from various holes and get some of the rust knocked off. Blow everything out real well. I used the Eastwood internal frame rust inhibitor https://www.eastwood.com/eastwood-in...z-aerosol.html
to shoot inside the enclosed areas of the frame through the various frame holes.
Then once the frame was done, gave it about 2 weeks to dry and then shot a coat of POR-15 top coat to seal everything. I then waited another 2 weeks or so as I worked on the frame and gave it a coat of Rustoleum black on top of that.
I don't see any use for soda blasting unless it was for thinner sheet or delicate parts. But, sure some would use soda blasting.
I would just make sure that the blaster you select if you have someone do it, does not use some real high pressure system that will destroy stuff. You don't want some guy who sandblasts boat hulls doing it.