Just like a girl I used to know always said, "Bigger is ALWAYS better" -- especially when it comes to antennas. I've said this before -- you can probably find it in the archives someplace.
The ideal MOBILE antenna for CB (or anything, for that matter) is a full quarter-wave antenna. For a CB that's be one of those 9-foot whips mounted on a spring on the bumper of your vehicle. Anything shorter than a quarter-wave is using something, like a coil, to shorten the physical length of the antenna in exchange for a reduction in efficiency (the radio MUST see a quarter-wave load to work best).
If you don't want a nine-foot antenna and choose to use one with a coil, chances are a bottom-mounted coil will work better than one in the middle of the antenna. Cellphone antennas have a coil in the middle -- but the wavelength there is WAY shorter than on CB, so the antenna can have improved electrical properties by using a coil in the middle.
If you want maximum efficiency out of your antenna, mount it permanently in the middle of the roof of your vehicle. You'll have the best possible ground plane -- the whole danged roof -- and the ground connection will be good, too. You can mount the antenna base through the roof and screw the coil/base onto that mount -- and then, for the 51 weeks that you don't use the antenna -- remove the coil/antenna and have nothing but a quarter-inch tall mount on the roof of the vehicle -- no 4' whip hitting the ceiling of the garage or getting snagged at the car wash.
I'm going to a picnic right away or I'd take a few photos of the mounting system I use -- but it takes too long and the hot dogs'll be on the grill soon. Maybe tomorrow, if you're interested.
Have a safe and happy Fourth of July, boys and girls.