I dont know how the flash GPS receiver works, but the garmin is defi a better thing, they have been making them for years for planes. I know some pilots even carry them, as there easier to use than the $10,000 Aircraft system fitted.
As for the antenna, for the best reception they should idealy be on the highest point of the car, i.e. roof (al'la BMW) but not many people want to put a hole in the top of their car. So on the dash is the next best thing, but if you think how far the signal has to travel, well theres not much signal strength by the time it gets to you. So line of sight is best, but we have got aircraft system to work in the hangar(steel clad). But the amplification will be better than in a handheld box.