"Which is preferred? A vehicle specific fender/hood bracket, or a SO-239 trunk lip mount used on the hood as shown in the attached picture? "
In my opinion, the mount that actually 'screws' to metal for support would be electrically better; however, the Diamond type clamp mounts you point to are pretty physically robust and work well for numerous hams I know.
But personally, I prefer the very low profile type lip mount made by Antenex at: http://www.antenex.com/index051206.htm
In either case, if you are talking VHF/UHF, the Motorola NMO type mount is the superior antenna base to have on whatever type base mount you use. Good luck.