I have both...
I use a Garmin Sat Nav in the Car if I am going long distances or don't know where I am going.
Both you have to mount anyway.. and most sat Navs come with a Window mount at least.
At this point I would still say built in Sat Nav's come first.
Then the dedicated Sat Nav's
Closely followed by the iPhone and the Tom Tom Cradle.
Followed last by iPhone on its own without a Tom Tom but with some other sort of mount.. although your GPS signal may not be that great (also check if you have a solar reflecting windscreen since then your possibly stuffed either way without an external antenna.
Regards PGre