I don't know how much of an "audiophile" you are, but vehicles have the worst possible enclosure to try to reproduce sound in. Having said that, the kick panel is the best spot to put a speaker in a vehicle. Choosing to put a speaker in the kick panel over the door likely comes down to knowing what you're probably supposed to be hearing and then wanting the best possible sound reproduction in a vehicle. But I wouldn't sweat it too much. There's a lot that goes into a stereo way before being able tell the difference between a properly done kick panel set up vs a properly done door set up.
To fully reproduce sound, the stereo needs to be able to produce low mid and high frequency, and ideally you'd have a different size speaker for each range of frequency. But...that's more along the audiophile train of thought.
In my opinion, if you like listening to music, then at minimum any stereo install requires amplified 6 1/2" components with a 10" sub. That will sound REALLY nice, and that is more than enough sound for the air space we're dealing with.
The smaller you go on the speaker, the better it reproduces vocally. The larger, but better it reproduces mid bass, and larger again will reproduce low bass.
The difference in mid bass between a 5 1/4" round speaker and a 6 1/2" round speaker is substantial. The difference in vocal reproduction is noticeable, but you're not leaving anything on the table by going with a 6 1/2. (audiophile mindset says run a 3 way...tweeter, 4 or 5" mid-high driver and a 6 or 7" mid-bass driver...but the 6 1/2" component set up is a really nice compromise compared to say a 5 1/4" component set up where you leave out a lot of midbass or a 3 way where it's pretty complicated and expensive)
I have run a 5 1/4" component set, and they do sound really nice and have super crisp and clear sound reproduction, they just lack on the mid bass department. On the song counting crows by mr jones... the 3 bass notes at the very beginning are totally gone in a 5 1/4" driver. You can hear them, but the 5 1/4" doesn't have the authority to make it sound like a bass note. Which leads to turning up the sub woofer frequency to reproduce that sound, and it doesn't reproduce the sound tight enough or precise enough to take the place of a 6 1/2" driver.
A 4" speaker is even more so better at vocal reproduction. Mid bass would be non existent.
6x9's are poor with vocal reproduction, but good mid bass.
I know what you're thinking... run a 4" in the dash and a 6x9 in the back.... but a matched 6 1/2" component set will blow that out of the water in sound reproduction.
a 10" woofer has the authority to re-produce a low bass note and let you know its there in the music, but won't ever get BOOMy loud and obnoxious. a 10" woofer is pretty mild in the boomy department, but really brings the sound full circle that is being produced by the mid driver. If you like a bit of boom sound and want the raw air movement...then I'd recommend a 12" sub.
The difference between amped and non is substantial in clarity and cleanly being able to go loud. The difference between a coaxial and component is substantial in clarity and musical separation.
And to recap, I don't know how much of an audiophile you are but if you literally just want to reproduce some music and are happy with basic radio quality music reproduction, literally just about any coaxial speaker powered by a head unit is going to do that. Placed anywhere in the truck. Done.
Don't overthink it if you fall into this category.
