I was talking to a guy at the Sony Store and he was all confused about different medias and formats and editting software.
Then that guy's an idiot.
I'm assuming you're going to be using this for filming while boarding as much as anything else, including all the shocks and abuse that comes with it.
Flash is the best simply because it has no moving parts. Jump, slam down, shake the shit out of it, flash can take it.
A HDD is a disc that spins, they can skip, and if you happen to jar it too hard while it's reading or writing, you can end up physically dragging the head across the disk and kiss your entire drive goodbye.
A miniDV is a cassette that records in digital format. I'm not too familiar with this, so I'm not going to comment other than to say it too has moving parts.
Cost per GB, however, Flash is probably the most expensive, although the prices of it have been dropping through the floor for the past few years.