Between trunk-mounting and roof-mounting a 2m/70cm antenna, both as through-hole NMO mounts, is there really that big a difference performance-wise? I mean, I know roof mounting is better, but let's face it: looks are a factor for some of us (and especially our significant others), and a trunk mount is much more understated than dead center of the roof. Plus, from a convenience standpoint, if you live in a city, it's nice to be able to park in a garage without removing the antenna.
I've seen the diagram from Larsen's catalog, but numbers don't tell the whole story. Especially not to a new ham like me. -3dB is half the signal strength gone, or put another way, just half an S-unit. I'm more curious about practical experiences (especially from people who have tried both). Plenty of people act like you're asking for trouble if you don't roof-mount, but truthfully, would I really notice those missing 3dB, and if so, would I mind it? Or will there be other effects that I'll mind?
If necessary, to give the question a little more scope, this is my first mobile installation (if it wasn't obvious). The antenna is a Larsen NMO-2/70B. I drive a 2014 Camry and live in the middle of Philly (think multistory buildings), but take that with a grain of salt. If I'm getting in my car at all, I'm pretty likely going out of town.