I mount it inside the top of the cab under the sunroof glass, again using a bit of Velcro.Both. Usually I stick it to a patch of velcro I have on the roof, but the last couple times I've used it I had it on the dash facing out the windscreen. Think the roof is better but you've got to remember to take it off when you finish...they don't like being left out in the rain!
It gets a better signal there than on the top of the dash, and it's safe from the rain.