Ken - All things the same (drivers, crossovers, etc.) coaxials will sound better than components because there are fewer problems with phase coherence and time alignment. Coaxial speakers are better at maintaining the time and phase relationship coming from the two drivers. That said, most speaker companies do not put high end drivers in coax speakers becauae of the marketing perception that components are superior. MB Quart used to make an excellent coax, as did Infinity. In high end studio monitors and pa systems, components are the norm for flexibility, power handling and ease of repair, but the coaxials are mechanically/theoretically superior.
If you go with components, place them as close together as possible for the best imaging. Placing the tweets high and the mids low is a common mistake which some think improves imaging. It does not, it merely raises the location cue of the tweets and makes it seem like the image is raised. It also increases listener fatigue over long periods of listening. When speakers are placed properly, it will sound like the drum set, not just the cymbals and "s" sounds, is placed on your hood and the singers head is in the middle of your windshield.
Location is critical to good imaging. The best location for your drivers is where there is less difference in the distance from your ears, that measurement is called the "delta". For instance, placing the drivers in the doors puts them closer to your ears, but the difference in distance from your head is greater for the left and right channel. To understand this, use a tape measure to check the distance from your left ear to the left door speaker, then measure the distance from your right ear to the right door and subtract the smaller number from the larger number, that is your "delta". Now do the same measurements to the kick panels and get that delta. I bet the delta to the kicks is a smaller number thant the delta to the doors. Smaller delta = better imaging. The kick panels are superior location because of this, assuming there is no weird blockage and I don't mean a persons legs. Do a Google search on the Speaker Works Buick, the winningest audio competitor of all time. These were the first guys to use a lot of Image Dynmamics stuff, pioneered the use of hornsunder the dash and converted Rane professional pa equipment (a middle level proaudio brand) for use in automobiles. http://www.termpro.com/showcars/terminator/ca1096.html
The best example of this in practice are the Image Dynamics horns that mount under the dash. Also, look at the placements of the drivers on high end sound quality, not SPL competitors and show cars, winners.
Also, if your rear fill speakers are too loud and have too much high frequency, they will detract from imaging. Remember the sound you hear live comes from the front and the sound from the rear is reflected off the back wall with reduced amplitude and high frequency content.
Unless you are in a Lexus or Benz, sound deadening is a great idea. I did an install in my 1994 Explorer Sport where I sprayed deadener (some blue stuff from Crutchfield) in the doors, quarter panels, tailgate and all over the floor. It was so much quieter in the car, the system sounded MUCH better. When I closed the door, it sounded like a Benz. Check out Dynamat and Mr. Gasket.
Generally, car audio is all about hype and fashion, it is not about high quality engineering and acoustic reproduction. I will hold Image Dynamics up as ONE of the exceptions. Sounds weird I know, but where else will people tell you that a system with a $ 200 stiffening capacitor sounds better than a $ 50 stiffening cap. Where 12v power supplies are concerned a 1 farad cap is a 1 farad cap. Period. Granted, there is value to some people in blinking lights, voltage readouts and cool looking stuff, but you asked an audio quality question, not a "cool points" question.
If you REALLY want to understand sound, go ge
), so all opinions and facts welcome 
