Coaxials have the tweeter and the mid on the same axis, i.e the tweeter is mounted in the middle of the mid. With components you get two seperate speakers meaning you get much better bass from the mids and the option to position the tweeters for a better sound.
Components are generally regarded to be better because of the better sound quality and the added flexibility you get from being able to move the tweeter around. Thats not to say that coaxials arent any good but if your spending upwards of £100 Id say go for components. If you do go for component speakers, they sound a lot better when running of an amp rather than the Head Unit.
Rick