Both have their good and bad points but I choose sand (garnet) over soda. Reason being, it will remove bog and rust completely and leave the steel etched and ready to receive paint/primer.
Soda, while being more mild and less chance of warping/damaging panels, leaves a film that then needs to be properly removed before you can paint over the surface.
If you find someone that knows what they are doing, they can do the job using garnet with minimal damage to panels, but even then, I would not do the bonnet (due to it being so large and thin) or the inside of the roof either.