soda will only take paint off ...not rust, not undercoating, not caulking ... its a very gentle blasting material ... sand is abrasive in comparison, I use crushed glass which is a little more effective then sand and black beauty will rip the hell out of the steel.
i use a harbor freight one i got off Craigslist for 50 dollars. it runs soda or sand. i run black blast "black beauty" sand from menards. it works great actually if you know what youre doing. you need to adjust the air settings properly and also you adjust the sand valve accordingly. ive never had issues with the black beauty ruining steel.
I have one of the 15 lb HF soda blasters in the blue bottle, but haven't used it yet. I also have one of the 40 lb HF sand blasters in the red bottle (like the one pictured above). The red sand blaster will get 'er done. I hope the blue soda blaster works as well for its purpose.
Last time I was at a VW class. There were several mechanics talking about using the HF soda blaster to clean carbon deposits on valvetrains. They claimed it took the carbon off without damaging anything else, and they were doing it with out removing the head.
I've got the plastic 50lb model from harbor freight. I've only run soda, but I love it. Quality is so-so, but as long as it works, I'm happy. Did have to tighten everything up and teflon tape all the connections but it's pretty straight forward design.