Aside from it being far easier to me to input my route, I just prefer to use GPS app on an iPad (or iPad mini) because it means I get a really big screen which can be very easily seen at just a glance and I can get voice cues in an ear bud even with the music cranked up.
Being able to put the antenna in a better location is great too.
It also means one less device to tote around and keep plugged in. I can use the iPad as my main internet access device and other tasks that a traditional GPS is useless for.