The answer isn't as simple as people make out. Both processes have thier upsides and downsides. Both processes can be done badly and even if done by experienced professionals can go wrong to some extent.
Firstly blasting
I'm fairly experienced in blasting panels and shells and can tell you it's not just a case of 'rough' blasters not giving a damn and warping panels as people may think. All blasting comes with a risk of warping. I'm careful and have had many panels warp unfortunately. The risk really comes down to the media and pressures used. You will probably hear of people using walnut shell or soda to blast cars to avoid damage.. great but it wont easily cut through layers of old filler and paint and certainly wont 'cut' the steel enough to remove rust. The reality is you will have to strip large panels by hand first and then find a blaster who is disciplined enough to not go near the cosmetic panels on the car. Even then its risky. It's not easy to do yourself let alone find someone else to trust with it! You will also find the car needs to be acid treated after blasting anyway if you really want to protect it from rust. Blasting might make it look like all the rust has gone but in reality it is still there deep in the 'pores' of the steel and can only effectively be neutralised with a phosphoric spray/dip.. (incidentally the same thing they will dip your car in to get rid of the rust). The main downside of blasting in my opinion tho is it simply cant get to alot of the surfaces! You cant blast inside box sections, seams etc!
Dipping i have less experience with on a 'whole shell' basis although i do use pretty much the same system for smaller parts. Absolute main benefit of dipping (as mentioned) is it gets to the whole shell! You need to make sure all box sections are drilled to allow the stripper and acid to easily and freely get to all areas (and drain from all areas!). As mentioned it will also chemically neutralise rust as well. The main problem with dipping as people allude to is the stripper soaking into seams and then are emerging after a car has been painted. Any buisness properly setup for this (I only know two) will do heat cycles and washes to mitigate that (note I say mitigate not totally eliminate).
I've got to say i am much more a fan of dipping than of blasting. Blasting works ok on stuff thats less cosmetic and delicate and that maybe you don't care so much about. Dipping comes with much much less risk of actual damage do stuff like car shells. Essentially the only thing your going to damage is paint and rust not the underlying fabric of the car! A bit of paint damage is easiky reversible. A bowed roof skin isnt. That said I do both. My Bedford van will be dipped when I get around to doing that. Trying the blast it would be disastrous as some panels are just so thin they wouldn't hold up. To blast it I'd have to start removing the wings (which you cant get replacements for!) It would be a nightmare. My transit chassis on the other hand was blasted.. its stout 2-3mm steel and it's just a transit chassis.
Also worth mentioning theres only really 2 proper places I know of doing dipping for shells. Surface processing in Dudley would be my go to. Envrio strip in Tamworth does similar but they use an oven based system to strip the paint to get rid of the paint work. (And alledgidly get rid of the risk of lost dipping paint damage) I've not felt with them tho. It seems a bit iffy to heat the shell to me? None the less I wouldn't trust any other company. Most bad experiences have come from people using companies thay just don't understand cars and don't handle them correctly.
Firstly blasting
I'm fairly experienced in blasting panels and shells and can tell you it's not just a case of 'rough' blasters not giving a damn and warping panels as people may think. All blasting comes with a risk of warping. I'm careful and have had many panels warp unfortunately. The risk really comes down to the media and pressures used. You will probably hear of people using walnut shell or soda to blast cars to avoid damage.. great but it wont easily cut through layers of old filler and paint and certainly wont 'cut' the steel enough to remove rust. The reality is you will have to strip large panels by hand first and then find a blaster who is disciplined enough to not go near the cosmetic panels on the car. Even then its risky. It's not easy to do yourself let alone find someone else to trust with it! You will also find the car needs to be acid treated after blasting anyway if you really want to protect it from rust. Blasting might make it look like all the rust has gone but in reality it is still there deep in the 'pores' of the steel and can only effectively be neutralised with a phosphoric spray/dip.. (incidentally the same thing they will dip your car in to get rid of the rust). The main downside of blasting in my opinion tho is it simply cant get to alot of the surfaces! You cant blast inside box sections, seams etc!
Dipping i have less experience with on a 'whole shell' basis although i do use pretty much the same system for smaller parts. Absolute main benefit of dipping (as mentioned) is it gets to the whole shell! You need to make sure all box sections are drilled to allow the stripper and acid to easily and freely get to all areas (and drain from all areas!). As mentioned it will also chemically neutralise rust as well. The main problem with dipping as people allude to is the stripper soaking into seams and then are emerging after a car has been painted. Any buisness properly setup for this (I only know two) will do heat cycles and washes to mitigate that (note I say mitigate not totally eliminate).
I've got to say i am much more a fan of dipping than of blasting. Blasting works ok on stuff thats less cosmetic and delicate and that maybe you don't care so much about. Dipping comes with much much less risk of actual damage do stuff like car shells. Essentially the only thing your going to damage is paint and rust not the underlying fabric of the car! A bit of paint damage is easiky reversible. A bowed roof skin isnt. That said I do both. My Bedford van will be dipped when I get around to doing that. Trying the blast it would be disastrous as some panels are just so thin they wouldn't hold up. To blast it I'd have to start removing the wings (which you cant get replacements for!) It would be a nightmare. My transit chassis on the other hand was blasted.. its stout 2-3mm steel and it's just a transit chassis.
Also worth mentioning theres only really 2 proper places I know of doing dipping for shells. Surface processing in Dudley would be my go to. Envrio strip in Tamworth does similar but they use an oven based system to strip the paint to get rid of the paint work. (And alledgidly get rid of the risk of lost dipping paint damage) I've not felt with them tho. It seems a bit iffy to heat the shell to me? None the less I wouldn't trust any other company. Most bad experiences have come from people using companies thay just don't understand cars and don't handle them correctly.