phatrs- Here`s one of those answers that might not seem like one!
IMO, you need to use whatever is required. Oh sheesh, how`s *that* for a nonanswer?!? Actually, it *is* my answer and the only one I can give in good conscious.
Remember that, *used properly*, clay really just shears off above-surface contamination. It doesn`t really "pull stuff out of the paint" to an appreciable degree. So maybe it`s enough, but maybe it`s not.
If there are bits of Ferrous Contamination (e.g., "rail dust") on, or more importantly *IN* the paint, then you oughta use a Ferrous Contamination Removal Product (i.e., IronX or the ValuGard "B" that I`d use). The contamination gets into the (perhaps microscopic) pores/microfissures in the paint and while claying will remove the contamiation that`s "on" the paint, it won`t get all of the stuff that`s down *in* the paint. That can result in stains "coming back" in the form of rust blooms. Then you *gotta* go back and use the chemical approach to truly solve the problem.
The same goes for other forms of contamination too, such as Acidic Contamination.
Ferrous contamination gets the lion`s share of attention (perhaps because chemicals to resolve that do the "color change" that is so appealing), but it`s not the only thing to consider. That Acidic Contamination (acid rain/fallout, bird bombs, etc.) can *really* get down into the paint and it can then cause even more serious issues than the ferrous stuff. And unlike little specks of rust that scream "Ferrous Contamination!", it might not show until it`s too late and you have issues like etched clearcoat.
For Acidic Contamination, a highly alkaline product (e.g., ValuGard`s "A") is what`s called for.
BUT, if neither form of contamination are an issue, then claying/etc. will quite possibly be perfectly sufficient.
So my take is to use what`s required for the job, something that`s hard to predict over the internet.