Things to keep in mind regarding any two-way radio antenna:
1. 1/4-wave antennas need a ground plane (the ground plane "reflection" fools the transmission line (coax) into thinking it's impedance-matched to a 50-ohm 1/2-wave dipole).
2. The best all-around (omnidirectional) radiation pattern is obtained with the antenna in the center of the roof.
3. Mirror-mount antennas intensify the radiation pattern to the front and rear of the vehicle, but are only effective if two are mounted 1/4-wavelength (9 feet for 27MHz CB) apart.
You see them often on trucks, which can get the 9 foot spacing, but they don't work on cars or SUVs. On cars, where the spacing is less than 1/4-wavelength, the radiation pattern is actually
shortened front and rear and decreases your range for talking with other CBers on the road.
4. A 1/4-wave antenna mounted in the bumper area with a mirror mount will have a poor, if any, ground plane. The radiation pattern will be extremely short to the sides and rear of the vehicle, with a higher "lobe" toward the front.
5. A 1/2-wave antenna mounted in the bumper area with a mirror mount will be fairly omnidirectional (1/2-wave antennas need no ground plane), but the signal toward the front will be slightly attenuated by the vehicle's metal body.
The general rule-of-thumb is to use a ground-plane antenna system wherever possible. This means a roof-mount antenna in the middle of the roof.
If you are leery of punching a hole in the roof and routing the coax cable in the headliner, I would suggest a magnetic-mount antenna. These actually work quite well, don't require any holes, and can be stored inside the vehicle to prevent theft.
{Edit} If you go with a magnetic-mount antenna, be sure to move it around once in a while, to give the paint on the roof a chance to age evenly...
More in-depth info can be obtained at
http://www.firestik.com/Tech_Docs.htm