They all have their down-sides (Hmmm...that'd be a good magazine article!)
Sand blasting can pit or warp the metal. A warped door skin or hood is junk.
Soda blasting is pretty thorough, but if it isn't neutralized properly, paint won't stick to it. I'm told by a guy who does it that soda will not remove rust (even surface rust), and it won't touch plastic filler either.
"Acid dipping" is actually sodium hydroxide (or should be--if they place you're using really is using acid, run away.) It's the most thorough of the three, but I've heard you can end up with black "yuck" dripping out of seams years later. It also COMPLETELY strips inside every joint and panel, so you've then got bare metal. It also needs to be completely neutralized (with water). Because it's hard to neutralize without submerging the car (which nobody does), it's likely you won't get full paint adhesion in every nook and cranny. It does not, however, continue to "eat" any of the metal.
The sodium hydroxide just dissolves paint, grease etc. It does not touch rust. However, most places then put it in a "derust" tank that runs a low-level electrical charge through the metal, breaking the chemical bond between rust and the parent metal. The sheetmetal comes out perfectly clean, with the surface as-delivered from the factory.
If I could do it any way I wanted, it's hard to beat the chemical dipping, but it is expensive. I'm going to soda blast the body of my Suburban, and sand blast the chassis.
-Brad
