components
pros - Components allow you to position the drivers separately and more carefully, which will give you greater control over imaging. Try to keep the mid and tweeter as close together as possible - this will make the two drivers sound more like a single speaker, and preferably in the same plane (pointing in the same direction). If your standard speaker locations are very low down in the car you can place the tweeters higher up to lift the sound stage.
cons - Components require more work to install as you will have to mount the tweeters separately, and most component kits also have separate crossovers. There is often very limited space to locate the crossovers and they require far more wiring. Components also require more power to drive them as there are 2 pairs of drivers and the crossovers also soak up quite a bit of power. Therefore to get the best results you will need to add an external amplifier.
coaxials pros - Coaxials are much simpler to install, usually just a case of removing the original speaker and screwing the new one in it's place. The two drivers are in exactly the same place therefore not separating the sound source. Coaxials are perfect for rear speakers as imaging is not so important. A high power head-unit normally provides enough power to adequately drive a pair of coaxials, therefore no separate amplifier is necessary.
cons - Tweeters cannot be moved therefore the imaging is restricted by the location, if the speaker location is low in the front door (as most cars are) the sound stage will be fairly low - We have not yet come across anybody with ears in their feet!!
