I'm now facing this choice too. My big problem is living in a street lined with very tall trees, trees that b*****d pigeons just love to sit in after they've had breakfast. At certain times of the year I end up at the hand car wash up to 4 times a week! So, would the dealer or the detailer guarantee their treatment against the ravages of bird s**t?
To be honest, my outgoing car has been shat on '000's of times in the almost 10 years I've had it, and apart from 1 small mark, all the others fade away within a week. Embarrassingly, I've never had the car polished by anyone and have always used hand car washes......life's to short for car washing.
That said, the photo's I've seen on here of cars that have been detailed, are making me think of doing the same. Although, street parking and kerbs are probably a bigger issue to think about