By leaving just one antenna switched on at a time I'm able to measure the VSWR of the individual dipole antennas.  The top trace is a set of UHF/VHF "rubber-duckies" and the bottom is a set of wire monopolies tuned for VHF.  I don't see a great deal of variation in performance with either antenna type and the performance is within a dB or so between antennas for a given frequency.    It would be great to flatten out the curve - need to research wideband antennas.

The PVC antenna mounting is too flimsy to mount on a car safely so looking into a fiberglass mount.  Maybe use wood for the fiberglass tube couplers.