horns
can be excellent. they can provide an excellent soundstage (eg: placement of instruments in the audio 'image'), and are usually very efficient, so require only small (less expensive) amps to run them, even loudly.
however, they can be difficult to install. they are low profile in height, but wide. the usual position is along the bottom edge of your dash. that's fine in lots of older cars that feature flat bottom dashlines, but can pose lots of difficulty in more modern cars. often it requires lots of custom work to make them fit securely and not look conspicuous. it may also necessitate the relocation of underdash wiring/components to make room for the 'compression driver'.
horns can be difficult to tune, to create a nice 'flat' response that sounds nice. as such, you will most likely require a fairly comprehensive equaliser (eg: 15+ band graphic EQ, preferably 30-band/ 1/3-octave), which adds to the expense.
horns can also sound quite 'bright' and in your face; which may or may not be to your liking.
but, if you're after something a little different, are willing to have some custom installation done, and want one of the best means to excellent sound staging, then horns could be the way to go. and if you know someone who has good experience with them and can help install them properly, that's a great bonus (not many people have experience with them).
image dynamics offer horns. not sure on pricing, so you'd have to find a local retailer (not that common) to check. it's not worth trying really cheap horns as they can sound pretty bad i believe.
image dynamics horns good luck