I read somewhere that garlic spray works as a cloaking device rather than a repellant. However, they were talking specifically about a certain insect that zeroes in on specific plants, like potato bugs on potatoes (in fact, that may have been the one). In essence, said bug can't find their corresponding host. I can't say I've ever seen aphids on garlic when I grew hundreds of them. But then, aphids are comparatively rare in my gardens anyway....
I would also suspect that softneck garlics would have more of the right stuff, over hardnecks. The bulbs, at least, tend to have much stronger garlicky flavor.
