Long form works best for:
* Simple to understand products which have difficult to achieve benefits (weight loss pills, real estate riches courses). The idea is that once you find an interested party, it is just a matter of convincing him that YOUR SOLUTION is THE SOLUTION.
* The purchase process is single stepped with low time and mental effort involved on the part of the purchaser. (e-download, shipped product).
* It is really hard to sell services with the long form, although its been done. I'd still like to see a good example of a successful longform that sells a service, and analyse it to death.
Short form works best for:
* Complex sales where a conversion means pre-qualification, a phone call, question answering, a customized result (think general contractor), identity check or some other gate to completing the sale. The idea is just to give them just enough benefits that they feel both enticed and think it is low risk to give you their contact information. Almost all B2B sales fit in this category.
* Monthly services with a freemium model, and any other low risk opportunity to kick the tires.
* Items with a low purchase point and low effort checkout (App Store game).
That said, there are always exceptions to the rule, and you should test both forms of communication if you are unsure.