I agree with CuteKinks. I think it depends on the style. If it's something that depends completely on the pins to hold the whole things, then I'd most likely use bobby pins. If it's just pinning random hair pieces to an already secure area, I'd use hairpins.

I really, really like hairpins and use them as much as possibly because they seem to give me less of a headache from all the pins in my head from an updo and can be just as secure as a bobby pin if used right. But no matter what, with any updo, I find that I'm usually using a combo of bobby pins and hair pins, depending on where it's going and what I need it to do.